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Found in /usr/share/perl/5.34/pod/perlfaq1.pod What is Perl? Perl is a high-level programming language with an eclectic heritage written by Larry Wall and a cast of thousands. Perl's process, file, and text manipulation facilities make it particularly well-suited for tasks involving quick prototyping, system utilities, software tools, system management tasks, database access, graphical programming, networking, and web programming. Perl derives from the ubiquitous C programming language and to a lesser extent from sed, awk, the Unix shell, and many other tools and languages. These strengths make it especially popular with web developers and system administrators. Mathematicians, geneticists, journalists, managers and many other people also use Perl. Who supports Perl? Who develops it? Why is it free? The original culture of the pre-populist Internet and the deeply-held beliefs of Perl's author, Larry Wall, gave rise to the free and open distribution policy of Perl. Perl is supported by its users. The core, the standard Perl library, the optional modules, and the documentation you're reading now were all written by volunteers. The core development team (known as the Perl Porters) are a group of highly altruistic individuals committed to producing better software for free than you could hope to purchase for money. You may snoop on pending developments via the archives <http://www.nntp.perl.org/group/perl.perl5.porters/> or you can subscribe to the mailing list by sending perl5-porters-subscribe AT perl.org a subscription request (an empty message with no subject is fine). While the GNU project includes Perl in its distributions, there's no such thing as "GNU Perl". Perl is not produced nor maintained by the Free Software Foundation. Perl's licensing terms are also more open than GNU software's tend to be. You can get commercial support of Perl if you wish, although for most users the informal support will more than suffice. See the answer to "Where can I buy a commercial version of Perl?" for more information. Which version of Perl should I use? (contributed by brian d foy with updates from others) There is often a matter of opinion and taste, and there isn't any one answer that fits everyone. In general, you want to use either the current stable release, or the stable release immediately prior to that one. Beyond that, you have to consider several things and decide which is best for you. * If things aren't broken, upgrading perl may break them (or at least issue new warnings). * The latest versions of perl have more bug fixes. * The latest versions of perl may contain performance improvements and features not present in older versions. There have been many changes in perl since perl5 was first introduced. * The Perl community is geared toward supporting the most recent releases, so you'll have an easier time finding help for those. * Older versions of perl may have security vulnerabilities, some of which are serious (see perlsec and search CVEs <https://cve.mitre.org/cgi-bin/cvekey.cgi?keyword=Perl> for more information). * The latest versions are probably the least deployed and widely tested, so you may want to wait a few months after their release and see what problems others have if you are risk averse. * The immediate, in addition to the current stable release, the previous stable release is maintained. See "MAINTENANCE AND SUPPORT" in perlpolicy for more information. * There are really two tracks of perl development: a maintenance version and an experimental version. The maintenance versions are stable, and have an even number as the minor release (i.e. perl5.24.x, where 24 is the minor release). The experimental versions may include features that don't make it into the stable versions, and have an odd number as the minor release (i.e. perl5.25.x, where 25 is the minor release). * You can consult releases <http://dev.perl.org/perl5> to determine the current stable release of Perl. What are Perl 4, Perl 5, or Raku (Perl 6)? In short, Perl 4 is the parent to both Perl 5 and Raku (formerly known as Perl 6). Perl 5 is the older sibling, and though they are different languages, someone who knows one will spot many similarities in the other. The number after Perl (i.e. the 5 after Perl 5) is the major release of the perl interpreter as well as the version of the language. Each major version has significant differences that earlier versions cannot support. The current major release of Perl is Perl 5, first released in 1994. It can run scripts from the previous major release, Perl 4 (March 1991), but has significant differences. Raku is a reinvention of Perl, a language in the same lineage but not compatible. The two are complementary, not mutually exclusive. Raku is not meant to replace Perl, and vice versa. See "What is Raku (Perl 6)?" below to find out more. See perlhist for a history of Perl revisions. What is Raku (Perl 6)? Raku (formerly known as Perl 6) was *originally* described as the community's rewrite of Perl, however as the language evolved, it became clear that it is a separate language, but in the same language family as Perl. Raku is not intended primarily as a replacement for Perl, but as its own thing - and libraries exist to allow you to call Perl code from Raku programs and vice versa. Contrary to popular belief, Raku and Perl peacefully coexist with one another. Raku has proven to be a fascinating source of ideas for those using Perl (the Moose object system is a well-known example). There is overlap in the communities, and this overlap fosters the tradition of sharing and borrowing that have been instrumental to Perl's success. For more about Raku see <https://www.raku.org/>. "We're really serious about reinventing everything that needs reinventing." --Larry Wall How stable is Perl? Production releases, which incorporate bug fixes and new functionality, are widely tested before release. Since the 5.000 release, we have averaged about one production release per year. The Perl development team occasionally make changes to the internal core of the language, but all possible efforts are made toward backward compatibility. How often are new versions of Perl released? Recently, the plan has been to release a new version of Perl roughly every April, but getting the release right is more important than sticking rigidly to a calendar date, so the release date is somewhat flexible. The historical release dates can be viewed at <http://www.cpan.org/src/README.html>. Even numbered minor versions (5.14, 5.16, 5.18) are production versions, and odd numbered minor versions (5.15, 5.17, 5.19) are development versions. Unless you want to try out an experimental feature, you probably never want to install a development version of Perl. The Perl development team are called Perl 5 Porters, and their organization is described at <http://perldoc.perl.org/perlpolicy.html>. The organizational rules really just boil down to one: Larry is always right, even when he was wrong. Is Perl difficult to learn? No, Perl is easy to start learning <http://learn.perl.org/> --and easy to keep learning. It looks like most programming languages you're likely to have experience with, so if you've ever written a C program, an awk script, a shell script, or even a BASIC program, you're already partway there. Most tasks only require a small subset of the Perl language. One of the guiding mottos for Perl development is "there's more than one way to do it" (TMTOWTDI, sometimes pronounced "tim toady"). Perl's learning curve is therefore shallow (easy to learn) and long (there's a whole lot you can do if you really want). Finally, because Perl is frequently (but not always, and certainly not by definition) an interpreted language, you can write your programs and test them without an intermediate compilation step, allowing you to experiment and test/debug quickly and easily. This ease of experimentation flattens the learning curve even more. Things that make Perl easier to learn: Unix experience, almost any kind of programming experience, an understanding of regular expressions, and the ability to understand other people's code. If there's something you need to do, then it's probably already been done, and a working example is usually available for free. Don't forget Perl modules, either. They're discussed in Part 3 of this FAQ, along with CPAN <http://www.cpan.org/>, which is discussed in Part 2. How does Perl compare with other languages like Java, Python, REXX, Scheme, or Tcl? Perl can be used for almost any coding problem, even ones which require integrating specialist C code for extra speed. As with any tool it can be used well or badly. Perl has many strengths, and a few weaknesses, precisely which areas are good and bad is often a personal choice. When choosing a language you should also be influenced by the resources <http://www.cpan.org/>, testing culture <http://www.cpantesters.org/> and community <http://www.perl.org/community.html> which surrounds it. For comparisons to a specific language it is often best to create a small project in both languages and compare the results, make sure to use all the resources <http://www.cpan.org/> of each language, as a language is far more than just it's syntax. Can I do [task] in Perl? Perl is flexible and extensible enough for you to use on virtually any task, from one-line file-processing tasks to large, elaborate systems. For many people, Perl serves as a great replacement for shell scripting. For others, it serves as a convenient, high-level replacement for most of what they'd program in low-level languages like C or C++. It's ultimately up to you (and possibly your management) which tasks you'll use Perl for and which you won't. If you have a library that provides an API, you can make any component of it available as just another Perl function or variable using a Perl extension written in C or C++ and dynamically linked into your main perl interpreter. You can also go the other direction, and write your main program in C or C++, and then link in some Perl code on the fly, to create a powerful application. See perlembed. That said, there will always be small, focused, special-purpose languages dedicated to a specific problem domain that are simply more convenient for certain kinds of problems. Perl tries to be all things to all people, but nothing special to anyone. Examples of specialized languages that come to mind include prolog and matlab. When shouldn't I program in Perl? One good reason is when you already have an existing application written in another language that's all done (and done well), or you have an application language specifically designed for a certain task (e.g. prolog, make). If you find that you need to speed up a specific part of a Perl application (not something you often need) you may want to use C, but you can access this from your Perl code with perlxs. What's the difference between "perl" and "Perl"? "Perl" is the name of the language. Only the "P" is capitalized. The name of the interpreter (the program which runs the Perl script) is "perl" with a lowercase "p". You may or may not choose to follow this usage. But never write "PERL", because perl is not an acronym. How can I convince others to use Perl? (contributed by brian d foy) Appeal to their self interest! If Perl is new (and thus scary) to them, find something that Perl can do to solve one of their problems. That might mean that Perl either saves them something (time, headaches, money) or gives them something (flexibility, power, testability). In general, the benefit of a language is closely related to the skill of the people using that language. If you or your team can be faster, better, and stronger through Perl, you'll deliver more value. Remember, people often respond better to what they get out of it. If you run into resistance, figure out what those people get out of the other choice and how Perl might satisfy that requirement. You don't have to worry about finding or paying for Perl; it's freely available and several popular operating systems come with Perl. Community support in places such as Perlmonks ( <http://www.perlmonks.com> ) and the various Perl mailing lists ( <http://lists.perl.org> ) means that you can usually get quick answers to your problems. Finally, keep in mind that Perl might not be the right tool for every job. You're a much better advocate if your claims are reasonable and grounded in reality. Dogmatically advocating anything tends to make people discount your message. Be honest about possible disadvantages to your choice of Perl since any choice has trade-offs. You might find these links useful: * <http://www.perl.org/about.html> * <http://perltraining.com.au/whyperl.html> Found in /usr/share/perl/5.34/pod/perlfaq2.pod What machines support Perl? Where do I get it? The standard release of Perl (the one maintained by the Perl development team) is distributed only in source code form. You can find the latest releases at <http://www.cpan.org/src/>. Perl builds and runs on a bewildering number of platforms. Virtually all known and current Unix derivatives are supported (perl's native platform), as are other systems like VMS, DOS, OS/2, Windows, QNX, BeOS, OS X, MPE/iX and the Amiga. Binary distributions for some proprietary platforms can be found <http://www.cpan.org/ports/> directory. Because these are not part of the standard distribution, they may and in fact do differ from the base perl port in a variety of ways. You'll have to check their respective release notes to see just what the differences are. These differences can be either positive (e.g. extensions for the features of the particular platform that are not supported in the source release of perl) or negative (e.g. might be based upon a less current source release of perl). How can I get a binary version of Perl? See CPAN Ports <http://www.cpan.org/ports/> I don't have a C compiler. How can I build my own Perl interpreter? For Windows, use a binary version of Perl, Strawberry Perl <http://strawberryperl.com/> and ActivePerl <http://www.activestate.com/activeperl> come with a bundled C compiler. Otherwise if you really do want to build Perl, you need to get a binary version of "gcc" for your system first. Use a search engine to find out how to do this for your operating system. I copied the Perl binary from one machine to another, but scripts don't work. That's probably because you forgot libraries, or library paths differ. You really should build the whole distribution on the machine it will eventually live on, and then type "make install". Most other approaches are doomed to failure. One simple way to check that things are in the right place is to print out the hard-coded @INC that perl looks through for libraries: % perl -le 'print for @INC' If this command lists any paths that don't exist on your system, then you may need to move the appropriate libraries to these locations, or create symbolic links, aliases, or shortcuts appropriately. @INC is also printed as part of the output of % perl -V You might also want to check out "How do I keep my own module/library directory?" in perlfaq8. I grabbed the sources and tried to compile but gdbm/dynamic loading/malloc/linking/... failed. How do I make it work? Read the INSTALL file, which is part of the source distribution. It describes in detail how to cope with most idiosyncrasies that the "Configure" script can't work around for any given system or architecture. What modules and extensions are available for Perl? What is CPAN? CPAN stands for Comprehensive Perl Archive Network, a multi-gigabyte archive replicated on hundreds of machines all over the world. CPAN contains tens of thousands of modules and extensions, source code and documentation, designed for *everything* from commercial database interfaces to keyboard/screen control and running large web sites. You can search CPAN on <http://metacpan.org>. The master web site for CPAN is <http://www.cpan.org/>, <http://www.cpan.org/SITES.html> lists all mirrors. See the CPAN FAQ at <http://www.cpan.org/misc/cpan-faq.html> for answers to the most frequently asked questions about CPAN. The Task::Kensho module has a list of recommended modules which you should review as a good starting point. Where can I get information on Perl? * <http://www.perl.org/> * <http://perldoc.perl.org/> * <http://learn.perl.org/> The complete Perl documentation is available with the Perl distribution. If you have Perl installed locally, you probably have the documentation installed as well: type "perldoc perl" in a terminal or view online <http://perldoc.perl.org/perl.html>. (Some operating system distributions may ship the documentation in a different package; for instance, on Debian, you need to install the "perl-doc" package.) Many good books have been written about Perl--see the section later in perlfaq2 for more details. What is perl.com? Perl Mongers? pm.org? perl.org? cpan.org? Perl.com <http://www.perl.com/> used to be part of the O'Reilly Network, a subsidiary of O'Reilly Media. Although it retains most of the original content from its O'Reilly Network, it is now hosted by The Perl Foundation <http://www.perlfoundation.org/>. The Perl Foundation is an advocacy organization for the Perl language which maintains the web site <http://www.perl.org/> as a general advocacy site for the Perl language. It uses the domain to provide general support services to the Perl community, including the hosting of mailing lists, web sites, and other services. There are also many other sub-domains for special topics like learning Perl and jobs in Perl, such as: * <http://www.perl.org/> * <http://learn.perl.org/> * <http://jobs.perl.org/> * <http://lists.perl.org/> Perl Mongers <http://www.pm.org/> uses the pm.org domain for services related to local Perl user groups, including the hosting of mailing lists and web sites. See the Perl Mongers web site <http://www.pm.org/> for more information about joining, starting, or requesting services for a Perl user group. CPAN, or the Comprehensive Perl Archive Network <http://www.cpan.org/>, is a replicated, worldwide repository of Perl software. See What is CPAN?. Perl Books There are many good books on Perl <http://www.perl.org/books/library.html>. Which magazines have Perl content? There's also *$foo Magazin*, a German magazine dedicated to Perl, at ( <http://www.foo-magazin.de> ). The *Perl-Zeitung* is another German-speaking magazine for Perl beginners (see <http://perl-zeitung.at.tf> ). Several Unix/Linux related magazines frequently include articles on Perl. Which Perl blogs should I read? Perl News <http://perlnews.org/> covers some of the major events in the Perl world, Perl Weekly <http://perlweekly.com/> is a weekly e-mail (and RSS feed) of hand-picked Perl articles. <http://blogs.perl.org/> hosts many Perl blogs, there are also several blog aggregators: Perlsphere <http://perlsphere.net/> and IronMan <http://ironman.enlightenedperl.org/> are two of them. What mailing lists are there for Perl? A comprehensive list of Perl-related mailing lists can be found at <http://lists.perl.org/> Where can I buy a commercial version of Perl? Perl already *is* commercial software: it has a license that you can grab and carefully read to your manager. It is distributed in releases and comes in well-defined packages. There is a very large and supportive user community and an extensive literature. If you still need commercial support ActiveState <http://www.activestate.com/activeperl> offers this. Found in /usr/share/perl/5.34/pod/perlfaq3.pod How can I use Perl interactively? The typical approach uses the Perl debugger, described in the perldebug(1) manpage, on an "empty" program, like this: perl -de 42 Now just type in any legal Perl code, and it will be immediately evaluated. You can also examine the symbol table, get stack backtraces, check variable values, set breakpoints, and other operations typically found in symbolic debuggers. You can also use Devel::REPL which is an interactive shell for Perl, commonly known as a REPL - Read, Evaluate, Print, Loop. It provides various handy features. How do I find which modules are installed on my system? From the command line, you can use the "cpan" command's "-l" switch: $ cpan -l You can also use "cpan"'s "-a" switch to create an autobundle file that "CPAN.pm" understands and can use to re-install every module: $ cpan -a Inside a Perl program, you can use the ExtUtils::Installed module to show all installed distributions, although it can take awhile to do its magic. The standard library which comes with Perl just shows up as "Perl" (although you can get those with Module::CoreList). use ExtUtils::Installed; my $inst = ExtUtils::Installed->new(); my @modules = $inst->modules(); If you want a list of all of the Perl module filenames, you can use File::Find::Rule: use File::Find::Rule; my @files = File::Find::Rule-> extras({follow => 1})-> file()-> name( '*.pm' )-> in( @INC ) ; If you do not have that module, you can do the same thing with File::Find which is part of the standard library: use File::Find; my @files; find( { wanted => sub { push @files, $File::Find::fullname if -f $File::Find::fullname && /\.pm$/ }, follow => 1, follow_skip => 2, }, @INC ); print join "\n", @files; If you simply need to check quickly to see if a module is available, you can check for its documentation. If you can read the documentation the module is most likely installed. If you cannot read the documentation, the module might not have any (in rare cases): $ perldoc Module::Name You can also try to include the module in a one-liner to see if perl finds it: $ perl -MModule::Name -e1 (If you don't receive a "Can't locate ... in @INC" error message, then Perl found the module name you asked for.) How do I debug my Perl programs? (contributed by brian d foy) Before you do anything else, you can help yourself by ensuring that you let Perl tell you about problem areas in your code. By turning on warnings and strictures, you can head off many problems before they get too big. You can find out more about these in strict and warnings. #!/usr/bin/perl use strict; use warnings; Beyond that, the simplest debugger is the "print" function. Use it to look at values as you run your program: print STDERR "The value is [$value]\n"; The Data::Dumper module can pretty-print Perl data structures: use Data::Dumper qw( Dumper ); print STDERR "The hash is " . Dumper( \%hash ) . "\n"; Perl comes with an interactive debugger, which you can start with the "-d" switch. It's fully explained in perldebug. If you'd like a graphical user interface and you have Tk, you can use "ptkdb". It's on CPAN and available for free. If you need something much more sophisticated and controllable, Leon Brocard's Devel::ebug (which you can call with the "-D" switch as "-Debug") gives you the programmatic hooks into everything you need to write your own (without too much pain and suffering). You can also use a commercial debugger such as Affrus (Mac OS X), Komodo from Activestate (Windows and Mac OS X), or EPIC (most platforms). How do I profile my Perl programs? (contributed by brian d foy, updated Fri Jul 25 12:22:26 PDT 2008) The "Devel" namespace has several modules which you can use to profile your Perl programs. The Devel::NYTProf (New York Times Profiler) does both statement and subroutine profiling. It's available from CPAN and you also invoke it with the "-d" switch: perl -d:NYTProf some_perl.pl It creates a database of the profile information that you can turn into reports. The "nytprofhtml" command turns the data into an HTML report similar to the Devel::Cover report: nytprofhtml You might also be interested in using the Benchmark to measure and compare code snippets. You can read more about profiling in *Programming Perl*, chapter 20, or *Mastering Perl*, chapter 5. perldebguts documents creating a custom debugger if you need to create a special sort of profiler. brian d foy describes the process in *The Perl Journal*, "Creating a Perl Debugger", <http://www.ddj.com/184404522> , and "Profiling in Perl" <http://www.ddj.com/184404580> . Perl.com has two interesting articles on profiling: "Profiling Perl", by Simon Cozens, <https://www.perl.com/pub/2004/06/25/profiling.html/> and "Debugging and Profiling mod_perl Applications", by Frank Wiles, <http://www.perl.com/pub/a/2006/02/09/debug_mod_perl.html> . Randal L. Schwartz writes about profiling in "Speeding up Your Perl Programs" for *Unix Review*, <http://www.stonehenge.com/merlyn/UnixReview/col49.html> , and "Profiling in Template Toolkit via Overriding" for *Linux Magazine*, <http://www.stonehenge.com/merlyn/LinuxMag/col75.html> . How do I cross-reference my Perl programs? The B::Xref module can be used to generate cross-reference reports for Perl programs. perl -MO=Xref[,OPTIONS] scriptname.plx Is there a pretty-printer (formatter) for Perl? Perl::Tidy comes with a perl script perltidy which indents and reformats Perl scripts to make them easier to read by trying to follow the rules of the perlstyle. If you write Perl, or spend much time reading Perl, you will probably find it useful. Of course, if you simply follow the guidelines in perlstyle, you shouldn't need to reformat. The habit of formatting your code as you write it will help prevent bugs. Your editor can and should help you with this. The perl-mode or newer cperl-mode for emacs can provide remarkable amounts of help with most (but not all) code, and even less programmable editors can provide significant assistance. Tom Christiansen and many other VI users swear by the following settings in vi and its clones: set ai sw=4 map! ^O {^M}^[O^T Put that in your .exrc file (replacing the caret characters with control characters) and away you go. In insert mode, ^T is for indenting, ^D is for undenting, and ^O is for blockdenting--as it were. A more complete example, with comments, can be found at <http://www.cpan.org/authors/id/T/TO/TOMC/scripts/toms.exrc.gz> Is there an IDE or Windows Perl Editor? Perl programs are just plain text, so any editor will do. If you're on Unix, you already have an IDE--Unix itself. The Unix philosophy is the philosophy of several small tools that each do one thing and do it well. It's like a carpenter's toolbox. If you want an IDE, check the following (in alphabetical order, not order of preference): Eclipse <http://e-p-i-c.sf.net/> The Eclipse Perl Integration Project integrates Perl editing/debugging with Eclipse. Enginsite <http://www.enginsite.com/> Perl Editor by EngInSite is a complete integrated development environment (IDE) for creating, testing, and debugging Perl scripts; the tool runs on Windows 9x/NT/2000/XP or later. IntelliJ IDEA <https://plugins.jetbrains.com/plugin/7796> Camelcade plugin provides Perl5 support in IntelliJ IDEA and other JetBrains IDEs. Kephra <http://kephra.sf.net> GUI editor written in Perl using wxWidgets and Scintilla with lots of smaller features. Aims for a UI based on Perl principles like TIMTOWTDI and "easy things should be easy, hard things should be possible". Komodo <http://www.ActiveState.com/Products/Komodo/> ActiveState's cross-platform (as of October 2004, that's Windows, Linux, and Solaris), multi-language IDE has Perl support, including a regular expression debugger and remote debugging. Notepad++ <http://notepad-plus.sourceforge.net/> Open Perl IDE <http://open-perl-ide.sourceforge.net/> Open Perl IDE is an integrated development environment for writing and debugging Perl scripts with ActiveState's ActivePerl distribution under Windows 95/98/NT/2000. OptiPerl <http://www.optiperl.com/> OptiPerl is a Windows IDE with simulated CGI environment, including debugger and syntax-highlighting editor. Padre <http://padre.perlide.org/> Padre is cross-platform IDE for Perl written in Perl using wxWidgets to provide a native look and feel. It's open source under the Artistic License. It is one of the newer Perl IDEs. PerlBuilder <http://www.solutionsoft.com/perl.htm> PerlBuilder is an integrated development environment for Windows that supports Perl development. visiPerl+ <http://helpconsulting.net/visiperl/index.html> From Help Consulting, for Windows. Visual Perl <http://www.activestate.com/Products/Visual_Perl/> Visual Perl is a Visual Studio.NET plug-in from ActiveState. Zeus <http://www.zeusedit.com/lookmain.html> Zeus for Windows is another Win32 multi-language editor/IDE that comes with support for Perl. For editors: if you're on Unix you probably have vi or a vi clone already, and possibly an emacs too, so you may not need to download anything. In any emacs the cperl-mode (M-x cperl-mode) gives you perhaps the best available Perl editing mode in any editor. If you are using Windows, you can use any editor that lets you work with plain text, such as NotePad or WordPad. Word processors, such as Microsoft Word or WordPerfect, typically do not work since they insert all sorts of behind-the-scenes information, although some allow you to save files as "Text Only". You can also download text editors designed specifically for programming, such as Textpad ( <http://www.textpad.com/> ) and UltraEdit ( <http://www.ultraedit.com/> ), among others. If you are using MacOS, the same concerns apply. MacPerl (for Classic environments) comes with a simple editor. Popular external editors are BBEdit ( <http://www.barebones.com/products/bbedit/> ) or Alpha ( <http://www.his.com/~jguyer/Alpha/Alpha8.html> ). MacOS X users can use Unix editors as well. GNU Emacs <http://www.gnu.org/software/emacs/windows/ntemacs.html> MicroEMACS <http://www.microemacs.de/> XEmacs <http://www.xemacs.org/Download/index.html> Jed <http://space.mit.edu/~davis/jed/> or a vi clone such as Vim <http://www.vim.org/> Vile <http://invisible-island.net/vile/vile.html> The following are Win32 multilanguage editor/IDEs that support Perl: MultiEdit <http://www.MultiEdit.com/> SlickEdit <http://www.slickedit.com/> ConTEXT <http://www.contexteditor.org/> There is also a toyedit Text widget based editor written in Perl that is distributed with the Tk module on CPAN. The ptkdb ( <http://ptkdb.sourceforge.net/> ) is a Perl/Tk-based debugger that acts as a development environment of sorts. Perl Composer ( <http://perlcomposer.sourceforge.net/> ) is an IDE for Perl/Tk GUI creation. In addition to an editor/IDE you might be interested in a more powerful shell environment for Win32. Your options include bash from the Cygwin package ( <http://cygwin.com/> ) zsh <http://www.zsh.org/> Cygwin is covered by the GNU General Public License (but that shouldn't matter for Perl use). Cygwin contains (in addition to the shell) a comprehensive set of standard Unix toolkit utilities. BBEdit and TextWrangler are text editors for OS X that have a Perl sensitivity mode ( <http://www.barebones.com/> ). Where can I get Perl macros for vi? For a complete version of Tom Christiansen's vi configuration file, see <http://www.cpan.org/authors/id/T/TO/TOMC/scripts/toms.exrc.gz> , the standard benchmark file for vi emulators. The file runs best with nvi, the current version of vi out of Berkeley, which incidentally can be built with an embedded Perl interpreter--see <http://www.cpan.org/src/misc/> . Where can I get perl-mode or cperl-mode for emacs? Since Emacs version 19 patchlevel 22 or so, there have been both a perl-mode.el and support for the Perl debugger built in. These should come with the standard Emacs 19 distribution. Note that the perl-mode of emacs will have fits with "main'foo" (single quote), and mess up the indentation and highlighting. You are probably using "main::foo" in new Perl code anyway, so this shouldn't be an issue. For CPerlMode, see <http://www.emacswiki.org/cgi-bin/wiki/CPerlMode> How can I use curses with Perl? The Curses module from CPAN provides a dynamically loadable object module interface to a curses library. A small demo can be found at the directory <http://www.cpan.org/authors/id/T/TO/TOMC/scripts/rep.gz> ; this program repeats a command and updates the screen as needed, rendering rep ps axu similar to top. How can I write a GUI (X, Tk, Gtk, etc.) in Perl? (contributed by Ben Morrow) There are a number of modules which let you write GUIs in Perl. Most GUI toolkits have a perl interface: an incomplete list follows. Tk This works under Unix and Windows, and the current version doesn't look half as bad under Windows as it used to. Some of the gui elements still don't 'feel' quite right, though. The interface is very natural and 'perlish', making it easy to use in small scripts that just need a simple gui. It hasn't been updated in a while. Wx This is a Perl binding for the cross-platform wxWidgets toolkit ( <http://www.wxwidgets.org> ). It works under Unix, Win32 and Mac OS X, using native widgets (Gtk under Unix). The interface follows the C++ interface closely, but the documentation is a little sparse for someone who doesn't know the library, mostly just referring you to the C++ documentation. Gtk and Gtk2 These are Perl bindings for the Gtk toolkit ( <http://www.gtk.org> ). The interface changed significantly between versions 1 and 2 so they have separate Perl modules. It runs under Unix, Win32 and Mac OS X (currently it requires an X server on Mac OS, but a 'native' port is underway), and the widgets look the same on every platform: i.e., they don't match the native widgets. As with Wx, the Perl bindings follow the C API closely, and the documentation requires you to read the C documentation to understand it. Win32::GUI This provides access to most of the Win32 GUI widgets from Perl. Obviously, it only runs under Win32, and uses native widgets. The Perl interface doesn't really follow the C interface: it's been made more Perlish, and the documentation is pretty good. More advanced stuff may require familiarity with the C Win32 APIs, or reference to MSDN. CamelBones CamelBones ( <http://camelbones.sourceforge.net> ) is a Perl interface to Mac OS X's Cocoa GUI toolkit, and as such can be used to produce native GUIs on Mac OS X. It's not on CPAN, as it requires frameworks that CPAN.pm doesn't know how to install, but installation is via the standard OSX package installer. The Perl API is, again, very close to the ObjC API it's wrapping, and the documentation just tells you how to translate from one to the other. Qt There is a Perl interface to TrollTech's Qt toolkit, but it does not appear to be maintained. Athena Sx is an interface to the Athena widget set which comes with X, but again it appears not to be much used nowadays. How can I make my Perl program run faster? The best way to do this is to come up with a better algorithm. This can often make a dramatic difference. Jon Bentley's book *Programming Pearls* (that's not a misspelling!) has some good tips on optimization, too. Advice on benchmarking boils down to: benchmark and profile to make sure you're optimizing the right part, look for better algorithms instead of microtuning your code, and when all else fails consider just buying faster hardware. You will probably want to read the answer to the earlier question "How do I profile my Perl programs?" if you haven't done so already. A different approach is to autoload seldom-used Perl code. See the AutoSplit and AutoLoader modules in the standard distribution for that. Or you could locate the bottleneck and think about writing just that part in C, the way we used to take bottlenecks in C code and write them in assembler. Similar to rewriting in C, modules that have critical sections can be written in C (for instance, the PDL module from CPAN). If you're currently linking your perl executable to a shared *libc.so*, you can often gain a 10-25% performance benefit by rebuilding it to link with a static libc.a instead. This will make a bigger perl executable, but your Perl programs (and programmers) may thank you for it. See the INSTALL file in the source distribution for more information. The undump program was an ancient attempt to speed up Perl program by storing the already-compiled form to disk. This is no longer a viable option, as it only worked on a few architectures, and wasn't a good solution anyway. How can I make my Perl program take less memory? When it comes to time-space tradeoffs, Perl nearly always prefers to throw memory at a problem. Scalars in Perl use more memory than strings in C, arrays take more than that, and hashes use even more. While there's still a lot to be done, recent releases have been addressing these issues. For example, as of 5.004, duplicate hash keys are shared amongst all hashes using them, so require no reallocation. In some cases, using substr() or vec() to simulate arrays can be highly beneficial. For example, an array of a thousand booleans will take at least 20,000 bytes of space, but it can be turned into one 125-byte bit vector--a considerable memory savings. The standard Tie::SubstrHash module can also help for certain types of data structure. If you're working with specialist data structures (matrices, for instance) modules that implement these in C may use less memory than equivalent Perl modules. Another thing to try is learning whether your Perl was compiled with the system malloc or with Perl's builtin malloc. Whichever one it is, try using the other one and see whether this makes a difference. Information about malloc is in the INSTALL file in the source distribution. You can find out whether you are using perl's malloc by typing "perl -V:usemymalloc". Of course, the best way to save memory is to not do anything to waste it in the first place. Good programming practices can go a long way toward this: Don't slurp! Don't read an entire file into memory if you can process it line by line. Or more concretely, use a loop like this: # # Good Idea # while (my $line = <$file_handle>) { # ... } instead of this: # # Bad Idea # my @data = <$file_handle>; foreach (@data) { # ... } When the files you're processing are small, it doesn't much matter which way you do it, but it makes a huge difference when they start getting larger. Use map and grep selectively Remember that both map and grep expect a LIST argument, so doing this: @wanted = grep {/pattern/} <$file_handle>; will cause the entire file to be slurped. For large files, it's better to loop: while (<$file_handle>) { push(@wanted, $_) if /pattern/; } Avoid unnecessary quotes and stringification Don't quote large strings unless absolutely necessary: my $copy = "$large_string"; makes 2 copies of $large_string (one for $copy and another for the quotes), whereas my $copy = $large_string; only makes one copy. Ditto for stringifying large arrays: { local $, = "\n"; print @big_array; } is much more memory-efficient than either print join "\n", @big_array; or { local $" = "\n"; print "@big_array"; } Pass by reference Pass arrays and hashes by reference, not by value. For one thing, it's the only way to pass multiple lists or hashes (or both) in a single call/return. It also avoids creating a copy of all the contents. This requires some judgement, however, because any changes will be propagated back to the original data. If you really want to mangle (er, modify) a copy, you'll have to sacrifice the memory needed to make one. Tie large variables to disk For "big" data stores (i.e. ones that exceed available memory) consider using one of the DB modules to store it on disk instead of in RAM. This will incur a penalty in access time, but that's probably better than causing your hard disk to thrash due to massive swapping. Is it safe to return a reference to local or lexical data? Yes. Perl's garbage collection system takes care of this so everything works out right. sub makeone { my @a = ( 1 .. 10 ); return \@a; } for ( 1 .. 10 ) { push @many, makeone(); } print $many[4][5], "\n"; print "@many\n"; How can I hide the source for my Perl program? Delete it. :-) Seriously, there are a number of (mostly unsatisfactory) solutions with varying levels of "security". First of all, however, you *can't* take away read permission, because the source code has to be readable in order to be compiled and interpreted. (That doesn't mean that a CGI script's source is readable by people on the web, though--only by people with access to the filesystem.) So you have to leave the permissions at the socially friendly 0755 level. Some people regard this as a security problem. If your program does insecure things and relies on people not knowing how to exploit those insecurities, it is not secure. It is often possible for someone to determine the insecure things and exploit them without viewing the source. Security through obscurity, the name for hiding your bugs instead of fixing them, is little security indeed. You can try using encryption via source filters (Starting from Perl 5.8 the Filter::Simple and Filter::Util::Call modules are included in the standard distribution), but any decent programmer will be able to decrypt it. You can try using the byte code compiler and interpreter described later in perlfaq3, but the curious might still be able to de-compile it. You can try using the native-code compiler described later, but crackers might be able to disassemble it. These pose varying degrees of difficulty to people wanting to get at your code, but none can definitively conceal it (true of every language, not just Perl). It is very easy to recover the source of Perl programs. You simply feed the program to the perl interpreter and use the modules in the B:: hierarchy. The B::Deparse module should be able to defeat most attempts to hide source. Again, this is not unique to Perl. If you're concerned about people profiting from your code, then the bottom line is that nothing but a restrictive license will give you legal security. License your software and pepper it with threatening statements like "This is unpublished proprietary software of XYZ Corp. Your access to it does not give you permission to use it blah blah blah." We are not lawyers, of course, so you should see a lawyer if you want to be sure your license's wording will stand up in court. How can I compile my Perl program into byte code or C? (contributed by brian d foy) In general, you can't do this. There are some things that may work for your situation though. People usually ask this question because they want to distribute their works without giving away the source code, and most solutions trade disk space for convenience. You probably won't see much of a speed increase either, since most solutions simply bundle a Perl interpreter in the final product (but see "How can I make my Perl program run faster?"). The Perl Archive Toolkit is Perl's analog to Java's JAR. It's freely available and on CPAN ( <https://metacpan.org/pod/PAR> ). There are also some commercial products that may work for you, although you have to buy a license for them. The Perl Dev Kit ( <http://www.activestate.com/Products/Perl_Dev_Kit/> ) from ActiveState can "Turn your Perl programs into ready-to-run executables for HP-UX, Linux, Solaris and Windows." Perl2Exe ( <http://www.indigostar.com/perl2exe.htm> ) is a command line program for converting perl scripts to executable files. It targets both Windows and Unix platforms. How can I get "#!perl" to work on [MS-DOS,NT,...]? For OS/2 just use extproc perl -S -your_switches as the first line in "*.cmd" file ("-S" due to a bug in cmd.exe's "extproc" handling). For DOS one should first invent a corresponding batch file and codify it in "ALTERNATE_SHEBANG" (see the dosish.h file in the source distribution for more information). The Win95/NT installation, when using the ActiveState port of Perl, will modify the Registry to associate the ".pl" extension with the perl interpreter. If you install another port, perhaps even building your own Win95/NT Perl from the standard sources by using a Windows port of gcc (e.g., with cygwin or mingw32), then you'll have to modify the Registry yourself. In addition to associating ".pl" with the interpreter, NT people can use: "SET PATHEXT=%PATHEXT%;.PL" to let them run the program "install-linux.pl" merely by typing "install-linux". Under "Classic" MacOS, a perl program will have the appropriate Creator and Type, so that double-clicking them will invoke the MacPerl application. Under Mac OS X, clickable apps can be made from any "#!" script using Wil Sanchez' DropScript utility: <http://www.wsanchez.net/software/> . *IMPORTANT!*: Whatever you do, PLEASE don't get frustrated, and just throw the perl interpreter into your cgi-bin directory, in order to get your programs working for a web server. This is an EXTREMELY big security risk. Take the time to figure out how to do it correctly. Can I write useful Perl programs on the command line? Yes. Read perlrun for more information. Some examples follow. (These assume standard Unix shell quoting rules.) # sum first and last fields perl -lane 'print $F[0] + $F[-1]' * # identify text files perl -le 'for(@ARGV) {print if -f && -T _}' * # remove (most) comments from C program perl -0777 -pe 's{/\*.*?\*/}{}gs' foo.c # make file a month younger than today, defeating reaper daemons perl -e '$X=24*60*60; utime(time(),time() + 30 * $X,@ARGV)' * # find first unused uid perl -le '$i++ while getpwuid($i); print $i' # display reasonable manpath echo $PATH | perl -nl -072 -e ' s![^/+]*$!man!&&-d&&!$s{$_}++&&push@m,$_;END{print"@m"}' OK, the last one was actually an Obfuscated Perl Contest entry. :-) Why don't Perl one-liners work on my DOS/Mac/VMS system? The problem is usually that the command interpreters on those systems have rather different ideas about quoting than the Unix shells under which the one-liners were created. On some systems, you may have to change single-quotes to double ones, which you must *NOT* do on Unix or Plan9 systems. You might also have to change a single % to a %%. For example: # Unix (including Mac OS X) perl -e 'print "Hello world\n"' # DOS, etc. perl -e "print \"Hello world\n\"" # Mac Classic print "Hello world\n" (then Run "Myscript" or Shift-Command-R) # MPW perl -e 'print "Hello world\n"' # VMS perl -e "print ""Hello world\n""" The problem is that none of these examples are reliable: they depend on the command interpreter. Under Unix, the first two often work. Under DOS, it's entirely possible that neither works. If 4DOS was the command shell, you'd probably have better luck like this: perl -e "print <Ctrl-x>"Hello world\n<Ctrl-x>"" Under the Mac, it depends which environment you are using. The MacPerl shell, or MPW, is much like Unix shells in its support for several quoting variants, except that it makes free use of the Mac's non-ASCII characters as control characters. Using qq(), q(), and qx(), instead of "double quotes", 'single quotes', and `backticks`, may make one-liners easier to write. There is no general solution to all of this. It is a mess. [Some of this answer was contributed by Kenneth Albanowski.] Where can I learn about CGI or Web programming in Perl? For modules, get the CGI or LWP modules from CPAN. For textbooks, see the two especially dedicated to web stuff in the question on books. For problems and questions related to the web, like "Why do I get 500 Errors" or "Why doesn't it run from the browser right when it runs fine on the command line", see the troubleshooting guides and references in perlfaq9 or in the CGI MetaFAQ: L<http://www.perl.org/CGI_MetaFAQ.html> Looking into <https://plackperl.org> and modern Perl web frameworks is highly recommended, though; web programming in Perl has evolved a long way from the old days of simple CGI scripts. Where can I learn about object-oriented Perl programming? A good place to start is perlootut, and you can use perlobj for reference. A good book on OO on Perl is the "Object-Oriented Perl" by Damian Conway from Manning Publications, or "Intermediate Perl" by Randal Schwartz, brian d foy, and Tom Phoenix from O'Reilly Media. Where can I learn about linking C with Perl? If you want to call C from Perl, start with perlxstut, moving on to perlxs, xsubpp, and perlguts. If you want to call Perl from C, then read perlembed, perlcall, and perlguts. Don't forget that you can learn a lot from looking at how the authors of existing extension modules wrote their code and solved their problems. You might not need all the power of XS. The Inline::C module lets you put C code directly in your Perl source. It handles all the magic to make it work. You still have to learn at least some of the perl API but you won't have to deal with the complexity of the XS support files. I've read perlembed, perlguts, etc., but I can't embed perl in my C program; what am I doing wrong? Download the ExtUtils::Embed kit from CPAN and run `make test'. If the tests pass, read the pods again and again and again. If they fail, submit a bug report to <https://github.com/Perl/perl5/issues> with the output of "make test TEST_VERBOSE=1" along with "perl -V". Found in /usr/share/perl/5.34/pod/perlfaq4.pod Why am I getting long decimals (eg, 19.9499999999999) instead of the numbers I should be getting (eg, 19.95)? For the long explanation, see David Goldberg's "What Every Computer Scientist Should Know About Floating-Point Arithmetic" (<http://web.cse.msu.edu/~cse320/Documents/FloatingPoint.pdf>). Internally, your computer represents floating-point numbers in binary. Digital (as in powers of two) computers cannot store all numbers exactly. Some real numbers lose precision in the process. This is a problem with how computers store numbers and affects all computer languages, not just Perl. perlnumber shows the gory details of number representations and conversions. To limit the number of decimal places in your numbers, you can use the "printf" or "sprintf" function. See "Floating-point Arithmetic" in perlop for more details. printf "%.2f", 10/3; my $number = sprintf "%.2f", 10/3; Why isn't my octal data interpreted correctly? (contributed by brian d foy) You're probably trying to convert a string to a number, which Perl only converts as a decimal number. When Perl converts a string to a number, it ignores leading spaces and zeroes, then assumes the rest of the digits are in base 10: my $string = '0644'; print $string + 0; # prints 644 print $string + 44; # prints 688, certainly not octal! This problem usually involves one of the Perl built-ins that has the same name a Unix command that uses octal numbers as arguments on the command line. In this example, "chmod" on the command line knows that its first argument is octal because that's what it does: %prompt> chmod 644 file If you want to use the same literal digits (644) in Perl, you have to tell Perl to treat them as octal numbers either by prefixing the digits with a 0 or using "oct": chmod( 0644, $filename ); # right, has leading zero chmod( oct(644), $filename ); # also correct The problem comes in when you take your numbers from something that Perl thinks is a string, such as a command line argument in @ARGV: chmod( $ARGV[0], $filename ); # wrong, even if "0644" chmod( oct($ARGV[0]), $filename ); # correct, treat string as octal You can always check the value you're using by printing it in octal notation to ensure it matches what you think it should be. Print it in octal and decimal format: printf "0%o %d", $number, $number; Does Perl have a round() function? What about ceil() and floor()? Trig functions? Remember that "int()" merely truncates toward 0. For rounding to a certain number of digits, "sprintf()" or "printf()" is usually the easiest route. printf("%.3f", 3.1415926535); # prints 3.142 The POSIX module (part of the standard Perl distribution) implements "ceil()", "floor()", and a number of other mathematical and trigonometric functions. use POSIX; my $ceil = ceil(3.5); # 4 my $floor = floor(3.5); # 3 In 5.000 to 5.003 perls, trigonometry was done in the Math::Complex module. With 5.004, the Math::Trig module (part of the standard Perl distribution) implements the trigonometric functions. Internally it uses the Math::Complex module and some functions can break out from the real axis into the complex plane, for example the inverse sine of 2. Rounding in financial applications can have serious implications, and the rounding method used should be specified precisely. In these cases, it probably pays not to trust whichever system of rounding is being used by Perl, but instead to implement the rounding function you need yourself. To see why, notice how you'll still have an issue on half-way-point alternation: for (my $i = -5; $i <= 5; $i += 0.5) { printf "%.0f ",$i } -5 -4 -4 -4 -3 -2 -2 -2 -1 -0 0 0 1 2 2 2 3 4 4 4 5 Don't blame Perl. It's the same as in C. IEEE says we have to do this. Perl numbers whose absolute values are integers under 2**31 (on 32-bit machines) will work pretty much like mathematical integers. Other numbers are not guaranteed. How do I multiply matrices? Use the Math::Matrix or Math::MatrixReal modules (available from CPAN) or the PDL extension (also available from CPAN). How can I output Roman numerals? Get the <http://www.cpan.org/modules/by-module/Roman> module. How do I find the current century or millennium? Use the following simple functions: sub get_century { return int((((localtime(shift || time))[5] + 1999))/100); } sub get_millennium { return 1+int((((localtime(shift || time))[5] + 1899))/1000); } On some systems, the POSIX module's "strftime()" function has been extended in a non-standard way to use a %C format, which they sometimes claim is the "century". It isn't, because on most such systems, this is only the first two digits of the four-digit year, and thus cannot be used to determine reliably the current century or millennium. How can I find the Julian Day? (contributed by brian d foy and Dave Cross) You can use the Time::Piece module, part of the Standard Library, which can convert a date/time to a Julian Day: $ perl -MTime::Piece -le 'print localtime->julian_day' 2455607.7959375 Or the modified Julian Day: $ perl -MTime::Piece -le 'print localtime->mjd' 55607.2961226851 Or even the day of the year (which is what some people think of as a Julian day): $ perl -MTime::Piece -le 'print localtime->yday' 45 You can also do the same things with the DateTime module: $ perl -MDateTime -le'print DateTime->today->jd' 2453401.5 $ perl -MDateTime -le'print DateTime->today->mjd' 53401 $ perl -MDateTime -le'print DateTime->today->doy' 31 You can use the Time::JulianDay module available on CPAN. Ensure that you really want to find a Julian day, though, as many people have different ideas about Julian days (see <http://www.hermetic.ch/cal_stud/jdn.htm> for instance): $ perl -MTime::JulianDay -le 'print local_julian_day( time )' 55608 Does Perl have a Year 2000 or 2038 problem? Is Perl Y2K compliant? (contributed by brian d foy) Perl itself never had a Y2K problem, although that never stopped people from creating Y2K problems on their own. See the documentation for "localtime" for its proper use. Starting with Perl 5.12, "localtime" and "gmtime" can handle dates past 03:14:08 January 19, 2038, when a 32-bit based time would overflow. You still might get a warning on a 32-bit "perl": % perl5.12 -E 'say scalar localtime( 0x9FFF_FFFFFFFF )' Integer overflow in hexadecimal number at -e line 1. Wed Nov 1 19:42:39 5576711 On a 64-bit "perl", you can get even larger dates for those really long running projects: % perl5.12 -E 'say scalar gmtime( 0x9FFF_FFFFFFFF )' Thu Nov 2 00:42:39 5576711 You're still out of luck if you need to keep track of decaying protons though. How do I validate input? (contributed by brian d foy) There are many ways to ensure that values are what you expect or want to accept. Besides the specific examples that we cover in the perlfaq, you can also look at the modules with "Assert" and "Validate" in their names, along with other modules such as Regexp::Common. Some modules have validation for particular types of input, such as Business::ISBN, Business::CreditCard, Email::Valid, and Data::Validate::IP. How do I expand function calls in a string? (contributed by brian d foy) This is documented in perlref, and although it's not the easiest thing to read, it does work. In each of these examples, we call the function inside the braces used to dereference a reference. If we have more than one return value, we can construct and dereference an anonymous array. In this case, we call the function in list context. print "The time values are @{ [localtime] }.\n"; If we want to call the function in scalar context, we have to do a bit more work. We can really have any code we like inside the braces, so we simply have to end with the scalar reference, although how you do that is up to you, and you can use code inside the braces. Note that the use of parens creates a list context, so we need "scalar" to force the scalar context on the function: print "The time is ${\(scalar localtime)}.\n" print "The time is ${ my $x = localtime; \$x }.\n"; If your function already returns a reference, you don't need to create the reference yourself. sub timestamp { my $t = localtime; \$t } print "The time is ${ timestamp() }.\n"; The "Interpolation" module can also do a lot of magic for you. You can specify a variable name, in this case "E", to set up a tied hash that does the interpolation for you. It has several other methods to do this as well. use Interpolation E => 'eval'; print "The time values are $E{localtime()}.\n"; In most cases, it is probably easier to simply use string concatenation, which also forces scalar context. print "The time is " . localtime() . ".\n"; How do I capitalize all the words on one line? (contributed by brian d foy) Damian Conway's Text::Autoformat handles all of the thinking for you. use Text::Autoformat; my $x = "Dr. Strangelove or: How I Learned to Stop ". "Worrying and Love the Bomb"; print $x, "\n"; for my $style (qw( sentence title highlight )) { print autoformat($x, { case => $style }), "\n"; } How do you want to capitalize those words? FRED AND BARNEY'S LODGE # all uppercase Fred And Barney's Lodge # title case Fred and Barney's Lodge # highlight case It's not as easy a problem as it looks. How many words do you think are in there? Wait for it... wait for it.... If you answered 5 you're right. Perl words are groups of "\w+", but that's not what you want to capitalize. How is Perl supposed to know not to capitalize that "s" after the apostrophe? You could try a regular expression: $string =~ s/ ( (^\w) #at the beginning of the line | # or (\s\w) #preceded by whitespace ) /\U$1/xg; $string =~ s/([\w']+)/\u\L$1/g; Now, what if you don't want to capitalize that "and"? Just use Text::Autoformat and get on with the next problem. :) How can I split a [character]-delimited string except when inside [character]? Several modules can handle this sort of parsing--Text::Balanced, Text::CSV, Text::CSV_XS, and Text::ParseWords, among others. Take the example case of trying to split a string that is comma-separated into its different fields. You can't use "split(/,/)" because you shouldn't split if the comma is inside quotes. For example, take a data line like this: SAR001,"","Cimetrix, Inc","Bob Smith","CAM",N,8,1,0,7,"Error, Core Dumped" Due to the restriction of the quotes, this is a fairly complex problem. Thankfully, we have Jeffrey Friedl, author of *Mastering Regular Expressions*, to handle these for us. He suggests (assuming your string is contained in $text): my @new = (); push(@new, $+) while $text =~ m{ "([^\"\\]*(?:\\.[^\"\\]*)*)",? # groups the phrase inside the quotes | ([^,]+),? | , }gx; push(@new, undef) if substr($text,-1,1) eq ','; If you want to represent quotation marks inside a quotation-mark-delimited field, escape them with backslashes (eg, "like \"this\"". Alternatively, the Text::ParseWords module (part of the standard Perl distribution) lets you say: use Text::ParseWords; @new = quotewords(",", 0, $text); For parsing or generating CSV, though, using Text::CSV rather than implementing it yourself is highly recommended; you'll save yourself odd bugs popping up later by just using code which has already been tried and tested in production for years. How do I strip blank space from the beginning/end of a string? (contributed by brian d foy) A substitution can do this for you. For a single line, you want to replace all the leading or trailing whitespace with nothing. You can do that with a pair of substitutions: s/^\s+//; s/\s+$//; You can also write that as a single substitution, although it turns out the combined statement is slower than the separate ones. That might not matter to you, though: s/^\s+|\s+$//g; In this regular expression, the alternation matches either at the beginning or the end of the string since the anchors have a lower precedence than the alternation. With the "/g" flag, the substitution makes all possible matches, so it gets both. Remember, the trailing newline matches the "\s+", and the "$" anchor can match to the absolute end of the string, so the newline disappears too. Just add the newline to the output, which has the added benefit of preserving "blank" (consisting entirely of whitespace) lines which the "^\s+" would remove all by itself: while( <> ) { s/^\s+|\s+$//g; print "$_\n"; } For a multi-line string, you can apply the regular expression to each logical line in the string by adding the "/m" flag (for "multi-line"). With the "/m" flag, the "$" matches *before* an embedded newline, so it doesn't remove it. This pattern still removes the newline at the end of the string: $string =~ s/^\s+|\s+$//gm; Remember that lines consisting entirely of whitespace will disappear, since the first part of the alternation can match the entire string and replace it with nothing. If you need to keep embedded blank lines, you have to do a little more work. Instead of matching any whitespace (since that includes a newline), just match the other whitespace: $string =~ s/^[\t\f ]+|[\t\f ]+$//mg; How do I pad a string with blanks or pad a number with zeroes? In the following examples, $pad_len is the length to which you wish to pad the string, $text or $num contains the string to be padded, and $pad_char contains the padding character. You can use a single character string constant instead of the $pad_char variable if you know what it is in advance. And in the same way you can use an integer in place of $pad_len if you know the pad length in advance. The simplest method uses the "sprintf" function. It can pad on the left or right with blanks and on the left with zeroes and it will not truncate the result. The "pack" function can only pad strings on the right with blanks and it will truncate the result to a maximum length of $pad_len. # Left padding a string with blanks (no truncation): my $padded = sprintf("%${pad_len}s", $text); my $padded = sprintf("%*s", $pad_len, $text); # same thing # Right padding a string with blanks (no truncation): my $padded = sprintf("%-${pad_len}s", $text); my $padded = sprintf("%-*s", $pad_len, $text); # same thing # Left padding a number with 0 (no truncation): my $padded = sprintf("%0${pad_len}d", $num); my $padded = sprintf("%0*d", $pad_len, $num); # same thing # Right padding a string with blanks using pack (will truncate): my $padded = pack("A$pad_len",$text); If you need to pad with a character other than blank or zero you can use one of the following methods. They all generate a pad string with the "x" operator and combine that with $text. These methods do not truncate $text. Left and right padding with any character, creating a new string: my $padded = $pad_char x ( $pad_len - length( $text ) ) . $text; my $padded = $text . $pad_char x ( $pad_len - length( $text ) ); Left and right padding with any character, modifying $text directly: substr( $text, 0, 0 ) = $pad_char x ( $pad_len - length( $text ) ); $text .= $pad_char x ( $pad_len - length( $text ) ); How do I extract selected columns from a string? (contributed by brian d foy) If you know the columns that contain the data, you can use "substr" to extract a single column. my $column = substr( $line, $start_column, $length ); You can use "split" if the columns are separated by whitespace or some other delimiter, as long as whitespace or the delimiter cannot appear as part of the data. my $line = ' fred barney betty '; my @columns = split /\s+/, $line; # ( '', 'fred', 'barney', 'betty' ); my $line = 'fred||barney||betty'; my @columns = split /\|/, $line; # ( 'fred', '', 'barney', '', 'betty' ); If you want to work with comma-separated values, don't do this since that format is a bit more complicated. Use one of the modules that handle that format, such as Text::CSV, Text::CSV_XS, or Text::CSV_PP. If you want to break apart an entire line of fixed columns, you can use "unpack" with the A (ASCII) format. By using a number after the format specifier, you can denote the column width. See the "pack" and "unpack" entries in perlfunc for more details. my @fields = unpack( $line, "A8 A8 A8 A16 A4" ); Note that spaces in the format argument to "unpack" do not denote literal spaces. If you have space separated data, you may want "split" instead. How do I find the soundex value of a string? (contributed by brian d foy) You can use the "Text::Soundex" module. If you want to do fuzzy or close matching, you might also try the String::Approx, and Text::Metaphone, and Text::DoubleMetaphone modules. How can I expand variables in text strings? (contributed by brian d foy) If you can avoid it, don't, or if you can use a templating system, such as Text::Template or Template Toolkit, do that instead. You might even be able to get the job done with "sprintf" or "printf": my $string = sprintf 'Say hello to %s and %s', $foo, $bar; However, for the one-off simple case where I don't want to pull out a full templating system, I'll use a string that has two Perl scalar variables in it. In this example, I want to expand $foo and $bar to their variable's values: my $foo = 'Fred'; my $bar = 'Barney'; $string = 'Say hello to $foo and $bar'; One way I can do this involves the substitution operator and a double "/e" flag. The first "/e" evaluates $1 on the replacement side and turns it into $foo. The second /e starts with $foo and replaces it with its value. $foo, then, turns into 'Fred', and that's finally what's left in the string: $string =~ s/(\$\w+)/$1/eeg; # 'Say hello to Fred and Barney' The "/e" will also silently ignore violations of strict, replacing undefined variable names with the empty string. Since I'm using the "/e" flag (twice even!), I have all of the same security problems I have with "eval" in its string form. If there's something odd in $foo, perhaps something like "@{[ system "rm -rf /" ]}", then I could get myself in trouble. To get around the security problem, I could also pull the values from a hash instead of evaluating variable names. Using a single "/e", I can check the hash to ensure the value exists, and if it doesn't, I can replace the missing value with a marker, in this case "???" to signal that I missed something: my $string = 'This has $foo and $bar'; my %Replacements = ( foo => 'Fred', ); # $string =~ s/\$(\w+)/$Replacements{$1}/g; $string =~ s/\$(\w+)/ exists $Replacements{$1} ? $Replacements{$1} : '???' /eg; print $string; Does Perl have anything like Ruby's #{} or Python's f string? Unlike the others, Perl allows you to embed a variable naked in a double quoted string, e.g. "variable $variable". When there isn't whitespace or other non-word characters following the variable name, you can add braces (e.g. "foo ${foo}bar") to ensure correct parsing. An array can also be embedded directly in a string, and will be expanded by default with spaces between the elements. The default LIST_SEPARATOR can be changed by assigning a different string to the special variable $", such as "local $" = ', ';". Perl also supports references within a string providing the equivalent of the features in the other two languages. "${\ ... }" embedded within a string will work for most simple statements such as an object->method call. More complex code can be wrapped in a do block "${\ do{...} }". When you want a list to be expanded per $", use "@{[ ... ]}". use Time::Piece; use Time::Seconds; my $scalar = 'STRING'; my @array = ( 'zorro', 'a', 1, 'B', 3 ); # Print the current date and time and then Tommorrow my $t = Time::Piece->new; say "Now is: ${\ $t->cdate() }"; say "Tomorrow: ${\ do{ my $T=Time::Piece->new + ONE_DAY ; $T->fullday }}"; # some variables in strings say "This is some scalar I have $scalar, this is an array @array."; say "You can also write it like this ${scalar} @{array}."; # Change the $LIST_SEPARATOR local $" = ':'; say "Set \$\" to delimit with ':' and sort the Array @{[ sort @array ]}"; You may also want to look at the module Quote::Code, and templating tools such as Template::Toolkit and Mojo::Template. See also: "How can I expand variables in text strings?" and "How do I expand function calls in a string?" in this FAQ. What's wrong with always quoting "$vars"? The problem is that those double-quotes force stringification--coercing numbers and references into strings--even when you don't want them to be strings. Think of it this way: double-quote expansion is used to produce new strings. If you already have a string, why do you need more? If you get used to writing odd things like these: print "$var"; # BAD my $new = "$old"; # BAD somefunc("$var"); # BAD You'll be in trouble. Those should (in 99.8% of the cases) be the simpler and more direct: print $var; my $new = $old; somefunc($var); Otherwise, besides slowing you down, you're going to break code when the thing in the scalar is actually neither a string nor a number, but a reference: func(\@array); sub func { my $aref = shift; my $oref = "$aref"; # WRONG } You can also get into subtle problems on those few operations in Perl that actually do care about the difference between a string and a number, such as the magical "++" autoincrement operator or the syscall() function. Stringification also destroys arrays. my @lines = `command`; print "@lines"; # WRONG - extra blanks print @lines; # right Why don't my <<HERE documents work? Here documents are found in perlop. Check for these three things: There must be no space after the << part. There (probably) should be a semicolon at the end of the opening token You can't (easily) have any space in front of the tag. There needs to be at least a line separator after the end token. If you want to indent the text in the here document, you can do this: # all in one (my $VAR = <<HERE_TARGET) =~ s/^\s+//gm; your text goes here HERE_TARGET But the HERE_TARGET must still be flush against the margin. If you want that indented also, you'll have to quote in the indentation. (my $quote = <<' FINIS') =~ s/^\s+//gm; ...we will have peace, when you and all your works have perished--and the works of your dark master to whom you would deliver us. You are a liar, Saruman, and a corrupter of men's hearts. --Theoden in /usr/src/perl/taint.c FINIS $quote =~ s/\s+--/\n--/; A nice general-purpose fixer-upper function for indented here documents follows. It expects to be called with a here document as its argument. It looks to see whether each line begins with a common substring, and if so, strips that substring off. Otherwise, it takes the amount of leading whitespace found on the first line and removes that much off each subsequent line. sub fix { local $_ = shift; my ($white, $leader); # common whitespace and common leading string if (/^\s*(?:([^\w\s]+)(\s*).*\n)(?:\s*\g1\g2?.*\n)+$/) { ($white, $leader) = ($2, quotemeta($1)); } else { ($white, $leader) = (/^(\s+)/, ''); } s/^\s*?$leader(?:$white)?//gm; return $_; } This works with leading special strings, dynamically determined: my $remember_the_main = fix<<' MAIN_INTERPRETER_LOOP'; @@@ int @@@ runops() { @@@ SAVEI32(runlevel); @@@ runlevel++; @@@ while ( op = (*op->op_ppaddr)() ); @@@ TAINT_NOT; @@@ return 0; @@@ } MAIN_INTERPRETER_LOOP Or with a fixed amount of leading whitespace, with remaining indentation correctly preserved: my $poem = fix<<EVER_ON_AND_ON; Now far ahead the Road has gone, And I must follow, if I can, Pursuing it with eager feet, Until it joins some larger way Where many paths and errands meet. And whither then? I cannot say. --Bilbo in /usr/src/perl/pp_ctl.c EVER_ON_AND_ON Beginning with Perl version 5.26, a much simpler and cleaner way to write indented here documents has been added to the language: the tilde (~) modifier. See "Indented Here-docs" in perlop for details. What is the difference between a list and an array? (contributed by brian d foy) A list is a fixed collection of scalars. An array is a variable that holds a variable collection of scalars. An array can supply its collection for list operations, so list operations also work on arrays: # slices ( 'dog', 'cat', 'bird' )[2,3]; @animals[2,3]; # iteration foreach ( qw( dog cat bird ) ) { ... } foreach ( @animals ) { ... } my @three = grep { length == 3 } qw( dog cat bird ); my @three = grep { length == 3 } @animals; # supply an argument list wash_animals( qw( dog cat bird ) ); wash_animals( @animals ); Array operations, which change the scalars, rearrange them, or add or subtract some scalars, only work on arrays. These can't work on a list, which is fixed. Array operations include "shift", "unshift", "push", "pop", and "splice". An array can also change its length: $#animals = 1; # truncate to two elements $#animals = 10000; # pre-extend to 10,001 elements You can change an array element, but you can't change a list element: $animals[0] = 'Rottweiler'; qw( dog cat bird )[0] = 'Rottweiler'; # syntax error! foreach ( @animals ) { s/^d/fr/; # works fine } foreach ( qw( dog cat bird ) ) { s/^d/fr/; # Error! Modification of read only value! } However, if the list element is itself a variable, it appears that you can change a list element. However, the list element is the variable, not the data. You're not changing the list element, but something the list element refers to. The list element itself doesn't change: it's still the same variable. You also have to be careful about context. You can assign an array to a scalar to get the number of elements in the array. This only works for arrays, though: my $count = @animals; # only works with arrays If you try to do the same thing with what you think is a list, you get a quite different result. Although it looks like you have a list on the righthand side, Perl actually sees a bunch of scalars separated by a comma: my $scalar = ( 'dog', 'cat', 'bird' ); # $scalar gets bird Since you're assigning to a scalar, the righthand side is in scalar context. The comma operator (yes, it's an operator!) in scalar context evaluates its lefthand side, throws away the result, and evaluates it's righthand side and returns the result. In effect, that list-lookalike assigns to $scalar it's rightmost value. Many people mess this up because they choose a list-lookalike whose last element is also the count they expect: my $scalar = ( 1, 2, 3 ); # $scalar gets 3, accidentally How can I remove duplicate elements from a list or array? (contributed by brian d foy) Use a hash. When you think the words "unique" or "duplicated", think "hash keys". If you don't care about the order of the elements, you could just create the hash then extract the keys. It's not important how you create that hash: just that you use "keys" to get the unique elements. my %hash = map { $_, 1 } @array; # or a hash slice: @hash{ @array } = (); # or a foreach: $hash{$_} = 1 foreach ( @array ); my @unique = keys %hash; If you want to use a module, try the "uniq" function from List::MoreUtils. In list context it returns the unique elements, preserving their order in the list. In scalar context, it returns the number of unique elements. use List::MoreUtils qw(uniq); my @unique = uniq( 1, 2, 3, 4, 4, 5, 6, 5, 7 ); # 1,2,3,4,5,6,7 my $unique = uniq( 1, 2, 3, 4, 4, 5, 6, 5, 7 ); # 7 You can also go through each element and skip the ones you've seen before. Use a hash to keep track. The first time the loop sees an element, that element has no key in %Seen. The "next" statement creates the key and immediately uses its value, which is "undef", so the loop continues to the "push" and increments the value for that key. The next time the loop sees that same element, its key exists in the hash *and* the value for that key is true (since it's not 0 or "undef"), so the next skips that iteration and the loop goes to the next element. my @unique = (); my %seen = (); foreach my $elem ( @array ) { next if $seen{ $elem }++; push @unique, $elem; } You can write this more briefly using a grep, which does the same thing. my %seen = (); my @unique = grep { ! $seen{ $_ }++ } @array; How can I tell whether a certain element is contained in a list or array? (portions of this answer contributed by Anno Siegel and brian d foy) Hearing the word "in" is an *in*dication that you probably should have used a hash, not a list or array, to store your data. Hashes are designed to answer this question quickly and efficiently. Arrays aren't. That being said, there are several ways to approach this. If you are going to make this query many times over arbitrary string values, the fastest way is probably to invert the original array and maintain a hash whose keys are the first array's values: my @blues = qw/azure cerulean teal turquoise lapis-lazuli/; my %is_blue = (); for (@blues) { $is_blue{$_} = 1 } Now you can check whether $is_blue{$some_color}. It might have been a good idea to keep the blues all in a hash in the first place. If the values are all small integers, you could use a simple indexed array. This kind of an array will take up less space: my @primes = (2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31); my @is_tiny_prime = (); for (@primes) { $is_tiny_prime[$_] = 1 } # or simply @istiny_prime[@primes] = (1) x @primes; Now you check whether $is_tiny_prime[$some_number]. If the values in question are integers instead of strings, you can save quite a lot of space by using bit strings instead: my @articles = ( 1..10, 150..2000, 2017 ); undef $read; for (@articles) { vec($read,$_,1) = 1 } Now check whether "vec($read,$n,1)" is true for some $n. These methods guarantee fast individual tests but require a re-organization of the original list or array. They only pay off if you have to test multiple values against the same array. If you are testing only once, the standard module List::Util exports the function "any" for this purpose. It works by stopping once it finds the element. It's written in C for speed, and its Perl equivalent looks like this subroutine: sub any (&@) { my $code = shift; foreach (@_) { return 1 if $code->(); } return 0; } If speed is of little concern, the common idiom uses grep in scalar context (which returns the number of items that passed its condition) to traverse the entire list. This does have the benefit of telling you how many matches it found, though. my $is_there = grep $_ eq $whatever, @array; If you want to actually extract the matching elements, simply use grep in list context. my @matches = grep $_ eq $whatever, @array; How do I test whether two arrays or hashes are equal? The following code works for single-level arrays. It uses a stringwise comparison, and does not distinguish defined versus undefined empty strings. Modify if you have other needs. $are_equal = compare_arrays(\@frogs, \@toads); sub compare_arrays { my ($first, $second) = @_; no warnings; # silence spurious -w undef complaints return 0 unless @$first == @$second; for (my $i = 0; $i < @$first; $i++) { return 0 if $first->[$i] ne $second->[$i]; } return 1; } For multilevel structures, you may wish to use an approach more like this one. It uses the CPAN module FreezeThaw: use FreezeThaw qw(cmpStr); my @a = my @b = ( "this", "that", [ "more", "stuff" ] ); printf "a and b contain %s arrays\n", cmpStr(\@a, \@b) == 0 ? "the same" : "different"; This approach also works for comparing hashes. Here we'll demonstrate two different answers: use FreezeThaw qw(cmpStr cmpStrHard); my %a = my %b = ( "this" => "that", "extra" => [ "more", "stuff" ] ); $a{EXTRA} = \%b; $b{EXTRA} = \%a; printf "a and b contain %s hashes\n", cmpStr(\%a, \%b) == 0 ? "the same" : "different"; printf "a and b contain %s hashes\n", cmpStrHard(\%a, \%b) == 0 ? "the same" : "different"; The first reports that both those the hashes contain the same data, while the second reports that they do not. Which you prefer is left as an exercise to the reader. How do I find the first array element for which a condition is true? To find the first array element which satisfies a condition, you can use the "first()" function in the List::Util module, which comes with Perl 5.8. This example finds the first element that contains "Perl". use List::Util qw(first); my $element = first { /Perl/ } @array; If you cannot use List::Util, you can make your own loop to do the same thing. Once you find the element, you stop the loop with last. my $found; foreach ( @array ) { if( /Perl/ ) { $found = $_; last } } If you want the array index, use the "firstidx()" function from "List::MoreUtils": use List::MoreUtils qw(firstidx); my $index = firstidx { /Perl/ } @array; Or write it yourself, iterating through the indices and checking the array element at each index until you find one that satisfies the condition: my( $found, $index ) = ( undef, -1 ); for( $i = 0; $i < @array; $i++ ) { if( $array[$i] =~ /Perl/ ) { $found = $array[$i]; $index = $i; last; } } How do I handle linked lists? (contributed by brian d foy) Perl's arrays do not have a fixed size, so you don't need linked lists if you just want to add or remove items. You can use array operations such as "push", "pop", "shift", "unshift", or "splice" to do that. Sometimes, however, linked lists can be useful in situations where you want to "shard" an array so you have many small arrays instead of a single big array. You can keep arrays longer than Perl's largest array index, lock smaller arrays separately in threaded programs, reallocate less memory, or quickly insert elements in the middle of the chain. Steve Lembark goes through the details in his YAPC::NA 2009 talk "Perly Linked Lists" ( <http://www.slideshare.net/lembark/perly-linked-lists> ), although you can just use his LinkedList::Single module. How do I handle circular lists? (contributed by brian d foy) If you want to cycle through an array endlessly, you can increment the index modulo the number of elements in the array: my @array = qw( a b c ); my $i = 0; while( 1 ) { print $array[ $i++ % @array ], "\n"; last if $i > 20; } You can also use Tie::Cycle to use a scalar that always has the next element of the circular array: use Tie::Cycle; tie my $cycle, 'Tie::Cycle', [ qw( FFFFFF 000000 FFFF00 ) ]; print $cycle; # FFFFFF print $cycle; # 000000 print $cycle; # FFFF00 The Array::Iterator::Circular creates an iterator object for circular arrays: use Array::Iterator::Circular; my $color_iterator = Array::Iterator::Circular->new( qw(red green blue orange) ); foreach ( 1 .. 20 ) { print $color_iterator->next, "\n"; } How do I shuffle an array randomly? If you either have Perl 5.8.0 or later installed, or if you have Scalar-List-Utils 1.03 or later installed, you can say: use List::Util 'shuffle'; @shuffled = shuffle(@list); If not, you can use a Fisher-Yates shuffle. sub fisher_yates_shuffle { my $deck = shift; # $deck is a reference to an array return unless @$deck; # must not be empty! my $i = @$deck; while (--$i) { my $j = int rand ($i+1); @$deck[$i,$j] = @$deck[$j,$i]; } } # shuffle my mpeg collection # my @mpeg = <audio/*/*.mp3>; fisher_yates_shuffle( \@mpeg ); # randomize @mpeg in place print @mpeg; Note that the above implementation shuffles an array in place, unlike the "List::Util::shuffle()" which takes a list and returns a new shuffled list. You've probably seen shuffling algorithms that work using splice, randomly picking another element to swap the current element with srand; @new = (); @old = 1 .. 10; # just a demo while (@old) { push(@new, splice(@old, rand @old, 1)); } This is bad because splice is already O(N), and since you do it N times, you just invented a quadratic algorithm; that is, O(N**2). This does not scale, although Perl is so efficient that you probably won't notice this until you have rather largish arrays. How do I process/modify each element of an array? Use "for"/"foreach": for (@lines) { s/foo/bar/; # change that word tr/XZ/ZX/; # swap those letters } Here's another; let's compute spherical volumes: my @volumes = @radii; for (@volumes) { # @volumes has changed parts $_ **= 3; $_ *= (4/3) * 3.14159; # this will be constant folded } which can also be done with "map()" which is made to transform one list into another: my @volumes = map {$_ ** 3 * (4/3) * 3.14159} @radii; If you want to do the same thing to modify the values of the hash, you can use the "values" function. As of Perl 5.6 the values are not copied, so if you modify $orbit (in this case), you modify the value. for my $orbit ( values %orbits ) { ($orbit **= 3) *= (4/3) * 3.14159; } Prior to perl 5.6 "values" returned copies of the values, so older perl code often contains constructions such as @orbits{keys %orbits} instead of "values %orbits" where the hash is to be modified. How do I select a random element from an array? Use the "rand()" function (see "rand" in perlfunc): my $index = rand @array; my $element = $array[$index]; Or, simply: my $element = $array[ rand @array ]; How do I permute N elements of a list? Use the List::Permutor module on CPAN. If the list is actually an array, try the Algorithm::Permute module (also on CPAN). It's written in XS code and is very efficient: use Algorithm::Permute; my @array = 'a'..'d'; my $p_iterator = Algorithm::Permute->new ( \@array ); while (my @perm = $p_iterator->next) { print "next permutation: (@perm)\n"; } For even faster execution, you could do: use Algorithm::Permute; my @array = 'a'..'d'; Algorithm::Permute::permute { print "next permutation: (@array)\n"; } @array; Here's a little program that generates all permutations of all the words on each line of input. The algorithm embodied in the "permute()" function is discussed in Volume 4 (still unpublished) of Knuth's *The Art of Computer Programming* and will work on any list: #!/usr/bin/perl -n # Fischer-Krause ordered permutation generator sub permute (&@) { my $code = shift; my @idx = 0..$#_; while ( $code->(@_[@idx]) ) { my $p = $#idx; --$p while $idx[$p-1] > $idx[$p]; my $q = $p or return; push @idx, reverse splice @idx, $p; ++$q while $idx[$p-1] > $idx[$q]; @idx[$p-1,$q]=@idx[$q,$p-1]; } } permute { print "@_\n" } split; The Algorithm::Loops module also provides the "NextPermute" and "NextPermuteNum" functions which efficiently find all unique permutations of an array, even if it contains duplicate values, modifying it in-place: if its elements are in reverse-sorted order then the array is reversed, making it sorted, and it returns false; otherwise the next permutation is returned. "NextPermute" uses string order and "NextPermuteNum" numeric order, so you can enumerate all the permutations of 0..9 like this: use Algorithm::Loops qw(NextPermuteNum); my @list= 0..9; do { print "@list\n" } while NextPermuteNum @list; How do I manipulate arrays of bits? Use "pack()" and "unpack()", or else "vec()" and the bitwise operations. For example, you don't have to store individual bits in an array (which would mean that you're wasting a lot of space). To convert an array of bits to a string, use "vec()" to set the right bits. This sets $vec to have bit N set only if $ints[N] was set: my @ints = (...); # array of bits, e.g. ( 1, 0, 0, 1, 1, 0 ... ) my $vec = ''; foreach( 0 .. $#ints ) { vec($vec,$_,1) = 1 if $ints[$_]; } The string $vec only takes up as many bits as it needs. For instance, if you had 16 entries in @ints, $vec only needs two bytes to store them (not counting the scalar variable overhead). Here's how, given a vector in $vec, you can get those bits into your @ints array: sub bitvec_to_list { my $vec = shift; my @ints; # Find null-byte density then select best algorithm if ($vec =~ tr/\0// / length $vec > 0.95) { use integer; my $i; # This method is faster with mostly null-bytes while($vec =~ /[^\0]/g ) { $i = -9 + 8 * pos $vec; push @ints, $i if vec($vec, ++$i, 1); push @ints, $i if vec($vec, ++$i, 1); push @ints, $i if vec($vec, ++$i, 1); push @ints, $i if vec($vec, ++$i, 1); push @ints, $i if vec($vec, ++$i, 1); push @ints, $i if vec($vec, ++$i, 1); push @ints, $i if vec($vec, ++$i, 1); push @ints, $i if vec($vec, ++$i, 1); } } else { # This method is a fast general algorithm use integer; my $bits = unpack "b*", $vec; push @ints, 0 if $bits =~ s/^(\d)// && $1; push @ints, pos $bits while($bits =~ /1/g); } return \@ints; } This method gets faster the more sparse the bit vector is. (Courtesy of Tim Bunce and Winfried Koenig.) You can make the while loop a lot shorter with this suggestion from Benjamin Goldberg: while($vec =~ /[^\0]+/g ) { push @ints, grep vec($vec, $_, 1), $-[0] * 8 .. $+[0] * 8; } Or use the CPAN module Bit::Vector: my $vector = Bit::Vector->new($num_of_bits); $vector->Index_List_Store(@ints); my @ints = $vector->Index_List_Read(); Bit::Vector provides efficient methods for bit vector, sets of small integers and "big int" math. Here's a more extensive illustration using vec(): # vec demo my $vector = "\xff\x0f\xef\xfe"; print "Ilya's string \\xff\\x0f\\xef\\xfe represents the number ", unpack("N", $vector), "\n"; my $is_set = vec($vector, 23, 1); print "Its 23rd bit is ", $is_set ? "set" : "clear", ".\n"; pvec($vector); set_vec(1,1,1); set_vec(3,1,1); set_vec(23,1,1); set_vec(3,1,3); set_vec(3,2,3); set_vec(3,4,3); set_vec(3,4,7); set_vec(3,8,3); set_vec(3,8,7); set_vec(0,32,17); set_vec(1,32,17); sub set_vec { my ($offset, $width, $value) = @_; my $vector = ''; vec($vector, $offset, $width) = $value; print "offset=$offset width=$width value=$value\n"; pvec($vector); } sub pvec { my $vector = shift; my $bits = unpack("b*", $vector); my $i = 0; my $BASE = 8; print "vector length in bytes: ", length($vector), "\n"; @bytes = unpack("A8" x length($vector), $bits); print "bits are: @bytes\n\n"; } What happens if I add or remove keys from a hash while iterating over it? (contributed by brian d foy) The easy answer is "Don't do that!" If you iterate through the hash with each(), you can delete the key most recently returned without worrying about it. If you delete or add other keys, the iterator may skip or double up on them since perl may rearrange the hash table. See the entry for "each()" in perlfunc. How do I look up a hash element by value? Create a reverse hash: my %by_value = reverse %by_key; my $key = $by_value{$value}; That's not particularly efficient. It would be more space-efficient to use: while (my ($key, $value) = each %by_key) { $by_value{$value} = $key; } If your hash could have repeated values, the methods above will only find one of the associated keys. This may or may not worry you. If it does worry you, you can always reverse the hash into a hash of arrays instead: while (my ($key, $value) = each %by_key) { push @{$key_list_by_value{$value}}, $key; } How do I sort a hash (optionally by value instead of key)? (contributed by brian d foy) To sort a hash, start with the keys. In this example, we give the list of keys to the sort function which then compares them ASCIIbetically (which might be affected by your locale settings). The output list has the keys in ASCIIbetical order. Once we have the keys, we can go through them to create a report which lists the keys in ASCIIbetical order. my @keys = sort { $a cmp $b } keys %hash; foreach my $key ( @keys ) { printf "%-20s %6d\n", $key, $hash{$key}; } We could get more fancy in the "sort()" block though. Instead of comparing the keys, we can compute a value with them and use that value as the comparison. For instance, to make our report order case-insensitive, we use "lc" to lowercase the keys before comparing them: my @keys = sort { lc $a cmp lc $b } keys %hash; Note: if the computation is expensive or the hash has many elements, you may want to look at the Schwartzian Transform to cache the computation results. If we want to sort by the hash value instead, we use the hash key to look it up. We still get out a list of keys, but this time they are ordered by their value. my @keys = sort { $hash{$a} <=> $hash{$b} } keys %hash; From there we can get more complex. If the hash values are the same, we can provide a secondary sort on the hash key. my @keys = sort { $hash{$a} <=> $hash{$b} or "\L$a" cmp "\L$b" } keys %hash; How can I always keep my hash sorted? You can look into using the "DB_File" module and "tie()" using the $DB_BTREE hash bindings as documented in "In Memory Databases" in DB_File. The Tie::IxHash module from CPAN might also be instructive. Although this does keep your hash sorted, you might not like the slowdown you suffer from the tie interface. Are you sure you need to do this? :) What's the difference between "delete" and "undef" with hashes? Hashes contain pairs of scalars: the first is the key, the second is the value. The key will be coerced to a string, although the value can be any kind of scalar: string, number, or reference. If a key $key is present in %hash, "exists($hash{$key})" will return true. The value for a given key can be "undef", in which case $hash{$key} will be "undef" while "exists $hash{$key}" will return true. This corresponds to ($key, "undef") being in the hash. Pictures help... Here's the %hash table: keys values +------+------+ | a | 3 | | x | 7 | | d | 0 | | e | 2 | +------+------+ And these conditions hold $hash{'a'} is true $hash{'d'} is false defined $hash{'d'} is true defined $hash{'a'} is true exists $hash{'a'} is true (Perl 5 only) grep ($_ eq 'a', keys %hash) is true If you now say undef $hash{'a'} your table now reads: keys values +------+------+ | a | undef| | x | 7 | | d | 0 | | e | 2 | +------+------+ and these conditions now hold; changes in caps: $hash{'a'} is FALSE $hash{'d'} is false defined $hash{'d'} is true defined $hash{'a'} is FALSE exists $hash{'a'} is true (Perl 5 only) grep ($_ eq 'a', keys %hash) is true Notice the last two: you have an undef value, but a defined key! Now, consider this: delete $hash{'a'} your table now reads: keys values +------+------+ | x | 7 | | d | 0 | | e | 2 | +------+------+ and these conditions now hold; changes in caps: $hash{'a'} is false $hash{'d'} is false defined $hash{'d'} is true defined $hash{'a'} is false exists $hash{'a'} is FALSE (Perl 5 only) grep ($_ eq 'a', keys %hash) is FALSE See, the whole entry is gone! How can I store a multidimensional array in a DBM file? Either stringify the structure yourself (no fun), or else get the MLDBM (which uses Data::Dumper) module from CPAN and layer it on top of either DB_File or GDBM_File. You might also try DBM::Deep, but it can be a bit slow. How can I make my hash remember the order I put elements into it? Use the Tie::IxHash from CPAN. use Tie::IxHash; tie my %myhash, 'Tie::IxHash'; for (my $i=0; $i<20; $i++) { $myhash{$i} = 2*$i; } my @keys = keys %myhash; # @keys = (0,1,2,3,...) Why does passing a subroutine an undefined element in a hash create it? (contributed by brian d foy) Are you using a really old version of Perl? Normally, accessing a hash key's value for a nonexistent key will *not* create the key. my %hash = (); my $value = $hash{ 'foo' }; print "This won't print\n" if exists $hash{ 'foo' }; Passing $hash{ 'foo' } to a subroutine used to be a special case, though. Since you could assign directly to $_[0], Perl had to be ready to make that assignment so it created the hash key ahead of time: my_sub( $hash{ 'foo' } ); print "This will print before 5.004\n" if exists $hash{ 'foo' }; sub my_sub { # $_[0] = 'bar'; # create hash key in case you do this 1; } Since Perl 5.004, however, this situation is a special case and Perl creates the hash key only when you make the assignment: my_sub( $hash{ 'foo' } ); print "This will print, even after 5.004\n" if exists $hash{ 'foo' }; sub my_sub { $_[0] = 'bar'; } However, if you want the old behavior (and think carefully about that because it's a weird side effect), you can pass a hash slice instead. Perl 5.004 didn't make this a special case: my_sub( @hash{ qw/foo/ } ); How can I make the Perl equivalent of a C structure/C++ class/hash or array of hashes or arrays? Usually a hash ref, perhaps like this: $record = { NAME => "Jason", EMPNO => 132, TITLE => "deputy peon", AGE => 23, SALARY => 37_000, PALS => [ "Norbert", "Rhys", "Phineas"], }; References are documented in perlref and perlreftut. Examples of complex data structures are given in perldsc and perllol. Examples of structures and object-oriented classes are in perlootut. How can I check if a key exists in a multilevel hash? (contributed by brian d foy) The trick to this problem is avoiding accidental autovivification. If you want to check three keys deep, you might na?vely try this: my %hash; if( exists $hash{key1}{key2}{key3} ) { ...; } Even though you started with a completely empty hash, after that call to "exists" you've created the structure you needed to check for "key3": %hash = ( 'key1' => { 'key2' => {} } ); That's autovivification. You can get around this in a few ways. The easiest way is to just turn it off. The lexical "autovivification" pragma is available on CPAN. Now you don't add to the hash: { no autovivification; my %hash; if( exists $hash{key1}{key2}{key3} ) { ...; } } The Data::Diver module on CPAN can do it for you too. Its "Dive" subroutine can tell you not only if the keys exist but also get the value: use Data::Diver qw(Dive); my @exists = Dive( \%hash, qw(key1 key2 key3) ); if( ! @exists ) { ...; # keys do not exist } elsif( ! defined $exists[0] ) { ...; # keys exist but value is undef } You can easily do this yourself too by checking each level of the hash before you move onto the next level. This is essentially what Data::Diver does for you: if( check_hash( \%hash, qw(key1 key2 key3) ) ) { ...; } sub check_hash { my( $hash, @keys ) = @_; return unless @keys; foreach my $key ( @keys ) { return unless eval { exists $hash->{$key} }; $hash = $hash->{$key}; } return 1; } How do I handle binary data correctly? Perl is binary-clean, so it can handle binary data just fine. On Windows or DOS, however, you have to use "binmode" for binary files to avoid conversions for line endings. In general, you should use "binmode" any time you want to work with binary data. Also see "binmode" in perlfunc or perlopentut. If you're concerned about 8-bit textual data then see perllocale. If you want to deal with multibyte characters, however, there are some gotchas. See the section on Regular Expressions. How do I determine whether a scalar is a number/whole/integer/float? Assuming that you don't care about IEEE notations like "NaN" or "Infinity", you probably just want to use a regular expression (see also perlretut and perlre): use 5.010; if ( /\D/ ) { say "\thas nondigits"; } if ( /^\d+\z/ ) { say "\tis a whole number"; } if ( /^-?\d+\z/ ) { say "\tis an integer"; } if ( /^[+-]?\d+\z/ ) { say "\tis a +/- integer"; } if ( /^-?(?:\d+\.?|\.\d)\d*\z/ ) { say "\tis a real number"; } if ( /^[+-]?(?=\.?\d)\d*\.?\d*(?:e[+-]?\d+)?\z/i ) { say "\tis a C float" } There are also some commonly used modules for the task. Scalar::Util (distributed with 5.8) provides access to perl's internal function "looks_like_number" for determining whether a variable looks like a number. Data::Types exports functions that validate data types using both the above and other regular expressions. Thirdly, there is Regexp::Common which has regular expressions to match various types of numbers. Those three modules are available from the CPAN. If you're on a POSIX system, Perl supports the "POSIX::strtod" function for converting strings to doubles (and also "POSIX::strtol" for longs). Its semantics are somewhat cumbersome, so here's a "getnum" wrapper function for more convenient access. This function takes a string and returns the number it found, or "undef" for input that isn't a C float. The "is_numeric" function is a front end to "getnum" if you just want to say, "Is this a float?" sub getnum { use POSIX qw(strtod); my $str = shift; $str =~ s/^\s+//; $str =~ s/\s+$//; $! = 0; my($num, $unparsed) = strtod($str); if (($str eq '') || ($unparsed != 0) || $!) { return undef; } else { return $num; } } sub is_numeric { defined getnum($_[0]) } Or you could check out the String::Scanf module on the CPAN instead. How do I keep persistent data across program calls? For some specific applications, you can use one of the DBM modules. See AnyDBM_File. More generically, you should consult the FreezeThaw or Storable modules from CPAN. Starting from Perl 5.8, Storable is part of the standard distribution. Here's one example using Storable's "store" and "retrieve" functions: use Storable; store(\%hash, "filename"); # later on... $href = retrieve("filename"); # by ref %hash = %{ retrieve("filename") }; # direct to hash How do I define methods for every class/object? (contributed by Ben Morrow) You can use the "UNIVERSAL" class (see UNIVERSAL). However, please be very careful to consider the consequences of doing this: adding methods to every object is very likely to have unintended consequences. If possible, it would be better to have all your object inherit from some common base class, or to use an object system like Moose that supports roles. How do I pack arrays of doubles or floats for XS code? The arrays.h/arrays.c code in the PGPLOT module on CPAN does just this. If you're doing a lot of float or double processing, consider using the PDL module from CPAN instead--it makes number-crunching easy. See <https://metacpan.org/release/PGPLOT> for the code. Found in /usr/share/perl/5.34/pod/perlfaq5.pod How do I flush/unbuffer an output filehandle? Why must I do this? (contributed by brian d foy) You might like to read Mark Jason Dominus's "Suffering From Buffering" at <http://perl.plover.com/FAQs/Buffering.html> . Perl normally buffers output so it doesn't make a system call for every bit of output. By saving up output, it makes fewer expensive system calls. For instance, in this little bit of code, you want to print a dot to the screen for every line you process to watch the progress of your program. Instead of seeing a dot for every line, Perl buffers the output and you have a long wait before you see a row of 50 dots all at once: # long wait, then row of dots all at once while( <> ) { print "."; print "\n" unless ++$count % 50; #... expensive line processing operations } To get around this, you have to unbuffer the output filehandle, in this case, "STDOUT". You can set the special variable $| to a true value (mnemonic: making your filehandles "piping hot"): $|++; # dot shown immediately while( <> ) { print "."; print "\n" unless ++$count % 50; #... expensive line processing operations } The $| is one of the per-filehandle special variables, so each filehandle has its own copy of its value. If you want to merge standard output and standard error for instance, you have to unbuffer each (although STDERR might be unbuffered by default): { my $previous_default = select(STDOUT); # save previous default $|++; # autoflush STDOUT select(STDERR); $|++; # autoflush STDERR, to be sure select($previous_default); # restore previous default } # now should alternate . and + while( 1 ) { sleep 1; print STDOUT "."; print STDERR "+"; print STDOUT "\n" unless ++$count % 25; } Besides the $| special variable, you can use "binmode" to give your filehandle a ":unix" layer, which is unbuffered: binmode( STDOUT, ":unix" ); while( 1 ) { sleep 1; print "."; print "\n" unless ++$count % 50; } For more information on output layers, see the entries for "binmode" and open in perlfunc, and the PerlIO module documentation. If you are using IO::Handle or one of its subclasses, you can call the "autoflush" method to change the settings of the filehandle: use IO::Handle; open my( $io_fh ), ">", "output.txt"; $io_fh->autoflush(1); The IO::Handle objects also have a "flush" method. You can flush the buffer any time you want without auto-buffering $io_fh->flush; How do I change, delete, or insert a line in a file, or append to the beginning of a file? (contributed by brian d foy) The basic idea of inserting, changing, or deleting a line from a text file involves reading and printing the file to the point you want to make the change, making the change, then reading and printing the rest of the file. Perl doesn't provide random access to lines (especially since the record input separator, $/, is mutable), although modules such as Tie::File can fake it. A Perl program to do these tasks takes the basic form of opening a file, printing its lines, then closing the file: open my $in, '<', $file or die "Can't read old file: $!"; open my $out, '>', "$file.new" or die "Can't write new file: $!"; while( <$in> ) { print $out $_; } close $out; Within that basic form, add the parts that you need to insert, change, or delete lines. To prepend lines to the beginning, print those lines before you enter the loop that prints the existing lines. open my $in, '<', $file or die "Can't read old file: $!"; open my $out, '>', "$file.new" or die "Can't write new file: $!"; print $out "# Add this line to the top\n"; # <--- HERE'S THE MAGIC while( <$in> ) { print $out $_; } close $out; To change existing lines, insert the code to modify the lines inside the "while" loop. In this case, the code finds all lowercased versions of "perl" and uppercases them. The happens for every line, so be sure that you're supposed to do that on every line! open my $in, '<', $file or die "Can't read old file: $!"; open my $out, '>', "$file.new" or die "Can't write new file: $!"; print $out "# Add this line to the top\n"; while( <$in> ) { s/\b(perl)\b/Perl/g; print $out $_; } close $out; To change only a particular line, the input line number, $., is useful. First read and print the lines up to the one you want to change. Next, read the single line you want to change, change it, and print it. After that, read the rest of the lines and print those: while( <$in> ) { # print the lines before the change print $out $_; last if $. == 4; # line number before change } my $line = <$in>; $line =~ s/\b(perl)\b/Perl/g; print $out $line; while( <$in> ) { # print the rest of the lines print $out $_; } To skip lines, use the looping controls. The "next" in this example skips comment lines, and the "last" stops all processing once it encounters either "__END__" or "__DATA__". while( <$in> ) { next if /^\s+#/; # skip comment lines last if /^__(END|DATA)__$/; # stop at end of code marker print $out $_; } Do the same sort of thing to delete a particular line by using "next" to skip the lines you don't want to show up in the output. This example skips every fifth line: while( <$in> ) { next unless $. % 5; print $out $_; } If, for some odd reason, you really want to see the whole file at once rather than processing line-by-line, you can slurp it in (as long as you can fit the whole thing in memory!): open my $in, '<', $file or die "Can't read old file: $!" open my $out, '>', "$file.new" or die "Can't write new file: $!"; my $content = do { local $/; <$in> }; # slurp! # do your magic here print $out $content; Modules such as Path::Tiny and Tie::File can help with that too. If you can, however, avoid reading the entire file at once. Perl won't give that memory back to the operating system until the process finishes. You can also use Perl one-liners to modify a file in-place. The following changes all 'Fred' to 'Barney' in inFile.txt, overwriting the file with the new contents. With the "-p" switch, Perl wraps a "while" loop around the code you specify with "-e", and "-i" turns on in-place editing. The current line is in $_. With "-p", Perl automatically prints the value of $_ at the end of the loop. See perlrun for more details. perl -pi -e 's/Fred/Barney/' inFile.txt To make a backup of "inFile.txt", give "-i" a file extension to add: perl -pi.bak -e 's/Fred/Barney/' inFile.txt To change only the fifth line, you can add a test checking $., the input line number, then only perform the operation when the test passes: perl -pi -e 's/Fred/Barney/ if $. == 5' inFile.txt To add lines before a certain line, you can add a line (or lines!) before Perl prints $_: perl -pi -e 'print "Put before third line\n" if $. == 3' inFile.txt You can even add a line to the beginning of a file, since the current line prints at the end of the loop: perl -pi -e 'print "Put before first line\n" if $. == 1' inFile.txt To insert a line after one already in the file, use the "-n" switch. It's just like "-p" except that it doesn't print $_ at the end of the loop, so you have to do that yourself. In this case, print $_ first, then print the line that you want to add. perl -ni -e 'print; print "Put after fifth line\n" if $. == 5' inFile.txt To delete lines, only print the ones that you want. perl -ni -e 'print if /d/' inFile.txt How do I count the number of lines in a file? (contributed by brian d foy) Conceptually, the easiest way to count the lines in a file is to simply read them and count them: my $count = 0; while( <$fh> ) { $count++; } You don't really have to count them yourself, though, since Perl already does that with the $. variable, which is the current line number from the last filehandle read: 1 while( <$fh> ); my $count = $.; If you want to use $., you can reduce it to a simple one-liner, like one of these: % perl -lne '} print $.; {' file % perl -lne 'END { print $. }' file Those can be rather inefficient though. If they aren't fast enough for you, you might just read chunks of data and count the number of newlines: my $lines = 0; open my($fh), '<:raw', $filename or die "Can't open $filename: $!"; while( sysread $fh, $buffer, 4096 ) { $lines += ( $buffer =~ tr/\n// ); } close $fh; However, that doesn't work if the line ending isn't a newline. You might change that "tr///" to a "s///" so you can count the number of times the input record separator, $/, shows up: my $lines = 0; open my($fh), '<:raw', $filename or die "Can't open $filename: $!"; while( sysread $fh, $buffer, 4096 ) { $lines += ( $buffer =~ s|$/||g; ); } close $fh; If you don't mind shelling out, the "wc" command is usually the fastest, even with the extra interprocess overhead. Ensure that you have an untainted filename though: #!perl -T $ENV{PATH} = undef; my $lines; if( $filename =~ /^([0-9a-z_.]+)\z/ ) { $lines = `/usr/bin/wc -l $1` chomp $lines; } How do I delete the last N lines from a file? (contributed by brian d foy) The easiest conceptual solution is to count the lines in the file then start at the beginning and print the number of lines (minus the last N) to a new file. Most often, the real question is how you can delete the last N lines without making more than one pass over the file, or how to do it without a lot of copying. The easy concept is the hard reality when you might have millions of lines in your file. One trick is to use File::ReadBackwards, which starts at the end of the file. That module provides an object that wraps the real filehandle to make it easy for you to move around the file. Once you get to the spot you need, you can get the actual filehandle and work with it as normal. In this case, you get the file position at the end of the last line you want to keep and truncate the file to that point: use File::ReadBackwards; my $filename = 'test.txt'; my $Lines_to_truncate = 2; my $bw = File::ReadBackwards->new( $filename ) or die "Could not read backwards in [$filename]: $!"; my $lines_from_end = 0; until( $bw->eof or $lines_from_end == $Lines_to_truncate ) { print "Got: ", $bw->readline; $lines_from_end++; } truncate( $filename, $bw->tell ); The File::ReadBackwards module also has the advantage of setting the input record separator to a regular expression. You can also use the Tie::File module which lets you access the lines through a tied array. You can use normal array operations to modify your file, including setting the last index and using "splice". How can I use Perl's "-i" option from within a program? "-i" sets the value of Perl's $^I variable, which in turn affects the behavior of "<>"; see perlrun for more details. By modifying the appropriate variables directly, you can get the same behavior within a larger program. For example: # ... { local($^I, @ARGV) = ('.orig', glob("*.c")); while (<>) { if ($. == 1) { print "This line should appear at the top of each file\n"; } s/\b(p)earl\b/${1}erl/i; # Correct typos, preserving case print; close ARGV if eof; # Reset $. } } # $^I and @ARGV return to their old values here This block modifies all the ".c" files in the current directory, leaving a backup of the original data from each file in a new ".c.orig" file. How can I copy a file? (contributed by brian d foy) Use the File::Copy module. It comes with Perl and can do a true copy across file systems, and it does its magic in a portable fashion. use File::Copy; copy( $original, $new_copy ) or die "Copy failed: $!"; If you can't use File::Copy, you'll have to do the work yourself: open the original file, open the destination file, then print to the destination file as you read the original. You also have to remember to copy the permissions, owner, and group to the new file. How do I make a temporary file name? If you don't need to know the name of the file, you can use "open()" with "undef" in place of the file name. In Perl 5.8 or later, the "open()" function creates an anonymous temporary file: open my $tmp, '+>', undef or die $!; Otherwise, you can use the File::Temp module. use File::Temp qw/ tempfile tempdir /; my $dir = tempdir( CLEANUP => 1 ); ($fh, $filename) = tempfile( DIR => $dir ); # or if you don't need to know the filename my $fh = tempfile( DIR => $dir ); The File::Temp has been a standard module since Perl 5.6.1. If you don't have a modern enough Perl installed, use the "new_tmpfile" class method from the IO::File module to get a filehandle opened for reading and writing. Use it if you don't need to know the file's name: use IO::File; my $fh = IO::File->new_tmpfile() or die "Unable to make new temporary file: $!"; If you're committed to creating a temporary file by hand, use the process ID and/or the current time-value. If you need to have many temporary files in one process, use a counter: BEGIN { use Fcntl; use File::Spec; my $temp_dir = File::Spec->tmpdir(); my $file_base = sprintf "%d-%d-0000", $$, time; my $base_name = File::Spec->catfile($temp_dir, $file_base); sub temp_file { my $fh; my $count = 0; until( defined(fileno($fh)) || $count++ > 100 ) { $base_name =~ s/-(\d+)$/"-" . (1 + $1)/e; # O_EXCL is required for security reasons. sysopen $fh, $base_name, O_WRONLY|O_EXCL|O_CREAT; } if( defined fileno($fh) ) { return ($fh, $base_name); } else { return (); } } } How can I manipulate fixed-record-length files? The most efficient way is using pack() and unpack(). This is faster than using substr() when taking many, many strings. It is slower for just a few. Here is a sample chunk of code to break up and put back together again some fixed-format input lines, in this case from the output of a normal, Berkeley-style ps: # sample input line: # 15158 p5 T 0:00 perl /home/tchrist/scripts/now-what my $PS_T = 'A6 A4 A7 A5 A*'; open my $ps, '-|', 'ps'; print scalar <$ps>; my @fields = qw( pid tt stat time command ); while (<$ps>) { my %process; @process{@fields} = unpack($PS_T, $_); for my $field ( @fields ) { print "$field: <$process{$field}>\n"; } print 'line=', pack($PS_T, @process{@fields} ), "\n"; } We've used a hash slice in order to easily handle the fields of each row. Storing the keys in an array makes it easy to operate on them as a group or loop over them with "for". It also avoids polluting the program with global variables and using symbolic references. How can I make a filehandle local to a subroutine? How do I pass filehandles between subroutines? How do I make an array of filehandles? As of perl5.6, open() autovivifies file and directory handles as references if you pass it an uninitialized scalar variable. You can then pass these references just like any other scalar, and use them in the place of named handles. open my $fh, $file_name; open local $fh, $file_name; print $fh "Hello World!\n"; process_file( $fh ); If you like, you can store these filehandles in an array or a hash. If you access them directly, they aren't simple scalars and you need to give "print" a little help by placing the filehandle reference in braces. Perl can only figure it out on its own when the filehandle reference is a simple scalar. my @fhs = ( $fh1, $fh2, $fh3 ); for( $i = 0; $i <= $#fhs; $i++ ) { print {$fhs[$i]} "just another Perl answer, \n"; } Before perl5.6, you had to deal with various typeglob idioms which you may see in older code. open FILE, "> $filename"; process_typeglob( *FILE ); process_reference( \*FILE ); sub process_typeglob { local *FH = shift; print FH "Typeglob!" } sub process_reference { local $fh = shift; print $fh "Reference!" } If you want to create many anonymous handles, you should check out the Symbol or IO::Handle modules. How can I use a filehandle indirectly? An indirect filehandle is the use of something other than a symbol in a place that a filehandle is expected. Here are ways to get indirect filehandles: $fh = SOME_FH; # bareword is strict-subs hostile $fh = "SOME_FH"; # strict-refs hostile; same package only $fh = *SOME_FH; # typeglob $fh = \*SOME_FH; # ref to typeglob (bless-able) $fh = *SOME_FH{IO}; # blessed IO::Handle from *SOME_FH typeglob Or, you can use the "new" method from one of the IO::* modules to create an anonymous filehandle and store that in a scalar variable. use IO::Handle; # 5.004 or higher my $fh = IO::Handle->new(); Then use any of those as you would a normal filehandle. Anywhere that Perl is expecting a filehandle, an indirect filehandle may be used instead. An indirect filehandle is just a scalar variable that contains a filehandle. Functions like "print", "open", "seek", or the "<FH>" diamond operator will accept either a named filehandle or a scalar variable containing one: ($ifh, $ofh, $efh) = (*STDIN, *STDOUT, *STDERR); print $ofh "Type it: "; my $got = <$ifh> print $efh "What was that: $got"; If you're passing a filehandle to a function, you can write the function in two ways: sub accept_fh { my $fh = shift; print $fh "Sending to indirect filehandle\n"; } Or it can localize a typeglob and use the filehandle directly: sub accept_fh { local *FH = shift; print FH "Sending to localized filehandle\n"; } Both styles work with either objects or typeglobs of real filehandles. (They might also work with strings under some circumstances, but this is risky.) accept_fh(*STDOUT); accept_fh($handle); In the examples above, we assigned the filehandle to a scalar variable before using it. That is because only simple scalar variables, not expressions or subscripts of hashes or arrays, can be used with built-ins like "print", "printf", or the diamond operator. Using something other than a simple scalar variable as a filehandle is illegal and won't even compile: my @fd = (*STDIN, *STDOUT, *STDERR); print $fd[1] "Type it: "; # WRONG my $got = <$fd[0]> # WRONG print $fd[2] "What was that: $got"; # WRONG With "print" and "printf", you get around this by using a block and an expression where you would place the filehandle: print { $fd[1] } "funny stuff\n"; printf { $fd[1] } "Pity the poor %x.\n", 3_735_928_559; # Pity the poor deadbeef. That block is a proper block like any other, so you can put more complicated code there. This sends the message out to one of two places: my $ok = -x "/bin/cat"; print { $ok ? $fd[1] : $fd[2] } "cat stat $ok\n"; print { $fd[ 1+ ($ok || 0) ] } "cat stat $ok\n"; This approach of treating "print" and "printf" like object methods calls doesn't work for the diamond operator. That's because it's a real operator, not just a function with a comma-less argument. Assuming you've been storing typeglobs in your structure as we did above, you can use the built-in function named "readline" to read a record just as "<>" does. Given the initialization shown above for @fd, this would work, but only because readline() requires a typeglob. It doesn't work with objects or strings, which might be a bug we haven't fixed yet. $got = readline($fd[0]); Let it be noted that the flakiness of indirect filehandles is not related to whether they're strings, typeglobs, objects, or anything else. It's the syntax of the fundamental operators. Playing the object game doesn't help you at all here. How can I open a filehandle to a string? (contributed by Peter J. Holzer, hjp-usenet2 AT hjp.at) Since Perl 5.8.0 a file handle referring to a string can be created by calling open with a reference to that string instead of the filename. This file handle can then be used to read from or write to the string: open(my $fh, '>', \$string) or die "Could not open string for writing"; print $fh "foo\n"; print $fh "bar\n"; # $string now contains "foo\nbar\n" open(my $fh, '<', \$string) or die "Could not open string for reading"; my $x = <$fh>; # $x now contains "foo\n" With older versions of Perl, the IO::String module provides similar functionality. How can I translate tildes (~) in a filename? Use the <> ("glob()") operator, documented in perlfunc. Versions of Perl older than 5.6 require that you have a shell installed that groks tildes. Later versions of Perl have this feature built in. The File::KGlob module (available from CPAN) gives more portable glob functionality. Within Perl, you may use this directly: $filename =~ s{ ^ ~ # find a leading tilde ( # save this in $1 [^/] # a non-slash character * # repeated 0 or more times (0 means me) ) }{ $1 ? (getpwnam($1))[7] : ( $ENV{HOME} || $ENV{LOGDIR} ) }ex; How come when I open a file read-write it wipes it out? Because you're using something like this, which truncates the file *then* gives you read-write access: open my $fh, '+>', '/path/name'; # WRONG (almost always) Whoops. You should instead use this, which will fail if the file doesn't exist: open my $fh, '+<', '/path/name'; # open for update Using ">" always clobbers or creates. Using "<" never does either. The "+" doesn't change this. Here are examples of many kinds of file opens. Those using "sysopen" all assume that you've pulled in the constants from Fcntl: use Fcntl; To open file for reading: open my $fh, '<', $path or die $!; sysopen my $fh, $path, O_RDONLY or die $!; To open file for writing, create new file if needed or else truncate old file: open my $fh, '>', $path or die $!; sysopen my $fh, $path, O_WRONLY|O_TRUNC|O_CREAT or die $!; sysopen my $fh, $path, O_WRONLY|O_TRUNC|O_CREAT, 0666 or die $!; To open file for writing, create new file, file must not exist: sysopen my $fh, $path, O_WRONLY|O_EXCL|O_CREAT or die $!; sysopen my $fh, $path, O_WRONLY|O_EXCL|O_CREAT, 0666 or die $!; To open file for appending, create if necessary: open my $fh, '>>', $path or die $!; sysopen my $fh, $path, O_WRONLY|O_APPEND|O_CREAT or die $!; sysopen my $fh, $path, O_WRONLY|O_APPEND|O_CREAT, 0666 or die $!; To open file for appending, file must exist: sysopen my $fh, $path, O_WRONLY|O_APPEND or die $!; To open file for update, file must exist: open my $fh, '+<', $path or die $!; sysopen my $fh, $path, O_RDWR or die $!; To open file for update, create file if necessary: sysopen my $fh, $path, O_RDWR|O_CREAT or die $!; sysopen my $fh, $path, O_RDWR|O_CREAT, 0666 or die $!; To open file for update, file must not exist: sysopen my $fh, $path, O_RDWR|O_EXCL|O_CREAT or die $!; sysopen my $fh, $path, O_RDWR|O_EXCL|O_CREAT, 0666 or die $!; To open a file without blocking, creating if necessary: sysopen my $fh, '/foo/somefile', O_WRONLY|O_NDELAY|O_CREAT or die "can't open /foo/somefile: $!": Be warned that neither creation nor deletion of files is guaranteed to be an atomic operation over NFS. That is, two processes might both successfully create or unlink the same file! Therefore O_EXCL isn't as exclusive as you might wish. See also perlopentut. Why do I sometimes get an "Argument list too long" when I use <*>? The "<>" operator performs a globbing operation (see above). In Perl versions earlier than v5.6.0, the internal glob() operator forks csh(1) to do the actual glob expansion, but csh can't handle more than 127 items and so gives the error message "Argument list too long". People who installed tcsh as csh won't have this problem, but their users may be surprised by it. To get around this, either upgrade to Perl v5.6.0 or later, do the glob yourself with readdir() and patterns, or use a module like File::Glob, one that doesn't use the shell to do globbing. How can I open a file named with a leading ">" or trailing blanks? (contributed by Brian McCauley) The special two-argument form of Perl's open() function ignores trailing blanks in filenames and infers the mode from certain leading characters (or a trailing "|"). In older versions of Perl this was the only version of open() and so it is prevalent in old code and books. Unless you have a particular reason to use the two-argument form you should use the three-argument form of open() which does not treat any characters in the filename as special. open my $fh, "<", " file "; # filename is " file " open my $fh, ">", ">file"; # filename is ">file" How can I reliably rename a file? If your operating system supports a proper mv(1) utility or its functional equivalent, this works: rename($old, $new) or system("mv", $old, $new); It may be more portable to use the File::Copy module instead. You just copy to the new file to the new name (checking return values), then delete the old one. This isn't really the same semantically as a "rename()", which preserves meta-information like permissions, timestamps, inode info, etc. How can I lock a file? Perl's builtin flock() function (see perlfunc for details) will call flock(2) if that exists, fcntl(2) if it doesn't (on perl version 5.004 and later), and lockf(3) if neither of the two previous system calls exists. On some systems, it may even use a different form of native locking. Here are some gotchas with Perl's flock(): 1 Produces a fatal error if none of the three system calls (or their close equivalent) exists. 2 lockf(3) does not provide shared locking, and requires that the filehandle be open for writing (or appending, or read/writing). 3 Some versions of flock() can't lock files over a network (e.g. on NFS file systems), so you'd need to force the use of fcntl(2) when you build Perl. But even this is dubious at best. See the flock entry of perlfunc and the INSTALL file in the source distribution for information on building Perl to do this. Two potentially non-obvious but traditional flock semantics are that it waits indefinitely until the lock is granted, and that its locks are *merely advisory*. Such discretionary locks are more flexible, but offer fewer guarantees. This means that files locked with flock() may be modified by programs that do not also use flock(). Cars that stop for red lights get on well with each other, but not with cars that don't stop for red lights. See the perlport manpage, your port's specific documentation, or your system-specific local manpages for details. It's best to assume traditional behavior if you're writing portable programs. (If you're not, you should as always feel perfectly free to write for your own system's idiosyncrasies (sometimes called "features"). Slavish adherence to portability concerns shouldn't get in the way of your getting your job done.) For more information on file locking, see also "File Locking" in perlopentut if you have it (new for 5.6). Why can't I just open(FH, ">file.lock")? A common bit of code NOT TO USE is this: sleep(3) while -e 'file.lock'; # PLEASE DO NOT USE open my $lock, '>', 'file.lock'; # THIS BROKEN CODE This is a classic race condition: you take two steps to do something which must be done in one. That's why computer hardware provides an atomic test-and-set instruction. In theory, this "ought" to work: sysopen my $fh, "file.lock", O_WRONLY|O_EXCL|O_CREAT or die "can't open file.lock: $!"; except that lamentably, file creation (and deletion) is not atomic over NFS, so this won't work (at least, not every time) over the net. Various schemes involving link() have been suggested, but these tend to involve busy-wait, which is also less than desirable. I still don't get locking. I just want to increment the number in the file. How can I do this? Didn't anyone ever tell you web-page hit counters were useless? They don't count number of hits, they're a waste of time, and they serve only to stroke the writer's vanity. It's better to pick a random number; they're more realistic. Anyway, this is what you can do if you can't help yourself. use Fcntl qw(:DEFAULT :flock); sysopen my $fh, "numfile", O_RDWR|O_CREAT or die "can't open numfile: $!"; flock $fh, LOCK_EX or die "can't flock numfile: $!"; my $num = <$fh> || 0; seek $fh, 0, 0 or die "can't rewind numfile: $!"; truncate $fh, 0 or die "can't truncate numfile: $!"; (print $fh $num+1, "\n") or die "can't write numfile: $!"; close $fh or die "can't close numfile: $!"; Here's a much better web-page hit counter: $hits = int( (time() - 850_000_000) / rand(1_000) ); If the count doesn't impress your friends, then the code might. :-) All I want to do is append a small amount of text to the end of a file. Do I still have to use locking? If you are on a system that correctly implements "flock" and you use the example appending code from "perldoc -f flock" everything will be OK even if the OS you are on doesn't implement append mode correctly (if such a system exists). So if you are happy to restrict yourself to OSs that implement "flock" (and that's not really much of a restriction) then that is what you should do. If you know you are only going to use a system that does correctly implement appending (i.e. not Win32) then you can omit the "seek" from the code in the previous answer. If you know you are only writing code to run on an OS and filesystem that does implement append mode correctly (a local filesystem on a modern Unix for example), and you keep the file in block-buffered mode and you write less than one buffer-full of output between each manual flushing of the buffer then each bufferload is almost guaranteed to be written to the end of the file in one chunk without getting intermingled with anyone else's output. You can also use the "syswrite" function which is simply a wrapper around your system's write(2) system call. There is still a small theoretical chance that a signal will interrupt the system-level "write()" operation before completion. There is also a possibility that some STDIO implementations may call multiple system level "write()"s even if the buffer was empty to start. There may be some systems where this probability is reduced to zero, and this is not a concern when using ":perlio" instead of your system's STDIO. How do I randomly update a binary file? If you're just trying to patch a binary, in many cases something as simple as this works: perl -i -pe 's{window manager}{window mangler}g' /usr/bin/emacs However, if you have fixed sized records, then you might do something more like this: my $RECSIZE = 220; # size of record, in bytes my $recno = 37; # which record to update open my $fh, '+<', 'somewhere' or die "can't update somewhere: $!"; seek $fh, $recno * $RECSIZE, 0; read $fh, $record, $RECSIZE == $RECSIZE or die "can't read record $recno: $!"; # munge the record seek $fh, -$RECSIZE, 1; print $fh $record; close $fh; Locking and error checking are left as an exercise for the reader. Don't forget them or you'll be quite sorry. How do I get a file's timestamp in perl? If you want to retrieve the time at which the file was last read, written, or had its meta-data (owner, etc) changed, you use the -A, -M, or -C file test operations as documented in perlfunc. These retrieve the age of the file (measured against the start-time of your program) in days as a floating point number. Some platforms may not have all of these times. See perlport for details. To retrieve the "raw" time in seconds since the epoch, you would call the stat function, then use "localtime()", "gmtime()", or "POSIX::strftime()" to convert this into human-readable form. Here's an example: my $write_secs = (stat($file))[9]; printf "file %s updated at %s\n", $file, scalar localtime($write_secs); If you prefer something more legible, use the File::stat module (part of the standard distribution in version 5.004 and later): # error checking left as an exercise for reader. use File::stat; use Time::localtime; my $date_string = ctime(stat($file)->mtime); print "file $file updated at $date_string\n"; The POSIX::strftime() approach has the benefit of being, in theory, independent of the current locale. See perllocale for details. How do I set a file's timestamp in perl? You use the utime() function documented in "utime" in perlfunc. By way of example, here's a little program that copies the read and write times from its first argument to all the rest of them. if (@ARGV < 2) { die "usage: cptimes timestamp_file other_files ...\n"; } my $timestamp = shift; my($atime, $mtime) = (stat($timestamp))[8,9]; utime $atime, $mtime, @ARGV; Error checking is, as usual, left as an exercise for the reader. The perldoc for utime also has an example that has the same effect as touch(1) on files that *already exist*. Certain file systems have a limited ability to store the times on a file at the expected level of precision. For example, the FAT and HPFS filesystem are unable to create dates on files with a finer granularity than two seconds. This is a limitation of the filesystems, not of utime(). How do I print to more than one file at once? To connect one filehandle to several output filehandles, you can use the IO::Tee or Tie::FileHandle::Multiplex modules. If you only have to do this once, you can print individually to each filehandle. for my $fh ($fh1, $fh2, $fh3) { print $fh "whatever\n" } How can I read in an entire file all at once? The customary Perl approach for processing all the lines in a file is to do so one line at a time: open my $input, '<', $file or die "can't open $file: $!"; while (<$input>) { chomp; # do something with $_ } close $input or die "can't close $file: $!"; This is tremendously more efficient than reading the entire file into memory as an array of lines and then processing it one element at a time, which is often--if not almost always--the wrong approach. Whenever you see someone do this: my @lines = <INPUT>; You should think long and hard about why you need everything loaded at once. It's just not a scalable solution. If you "mmap" the file with the File::Map module from CPAN, you can virtually load the entire file into a string without actually storing it in memory: use File::Map qw(map_file); map_file my $string, $filename; Once mapped, you can treat $string as you would any other string. Since you don't necessarily have to load the data, mmap-ing can be very fast and may not increase your memory footprint. You might also find it more fun to use the standard Tie::File module, or the DB_File module's $DB_RECNO bindings, which allow you to tie an array to a file so that accessing an element of the array actually accesses the corresponding line in the file. If you want to load the entire file, you can use the Path::Tiny module to do it in one simple and efficient step: use Path::Tiny; my $all_of_it = path($filename)->slurp; # entire file in scalar my @all_lines = path($filename)->lines; # one line per element Or you can read the entire file contents into a scalar like this: my $var; { local $/; open my $fh, '<', $file or die "can't open $file: $!"; $var = <$fh>; } That temporarily undefs your record separator, and will automatically close the file at block exit. If the file is already open, just use this: my $var = do { local $/; <$fh> }; You can also use a localized @ARGV to eliminate the "open": my $var = do { local( @ARGV, $/ ) = $file; <> }; For ordinary files you can also use the "read" function. read( $fh, $var, -s $fh ); That third argument tests the byte size of the data on the $fh filehandle and reads that many bytes into the buffer $var. How can I read in a file by paragraphs? Use the $/ variable (see perlvar for details). You can either set it to "" to eliminate empty paragraphs ("abc\n\n\n\ndef", for instance, gets treated as two paragraphs and not three), or "\n\n" to accept empty paragraphs. Note that a blank line must have no blanks in it. Thus "fred\n \nstuff\n\n" is one paragraph, but "fred\n\nstuff\n\n" is two. How can I read a single character from a file? From the keyboard? You can use the builtin "getc()" function for most filehandles, but it won't (easily) work on a terminal device. For STDIN, either use the Term::ReadKey module from CPAN or use the sample code in "getc" in perlfunc. If your system supports the portable operating system programming interface (POSIX), you can use the following code, which you'll note turns off echo processing as well. #!/usr/bin/perl -w use strict; $| = 1; for (1..4) { print "gimme: "; my $got = getone(); print "--> $got\n"; } exit; BEGIN { use POSIX qw(:termios_h); my ($term, $oterm, $echo, $noecho, $fd_stdin); my $fd_stdin = fileno(STDIN); $term = POSIX::Termios->new(); $term->getattr($fd_stdin); $oterm = $term->getlflag(); $echo = ECHO | ECHOK | ICANON; $noecho = $oterm & ~$echo; sub cbreak { $term->setlflag($noecho); $term->setcc(VTIME, 1); $term->setattr($fd_stdin, TCSANOW); } sub cooked { $term->setlflag($oterm); $term->setcc(VTIME, 0); $term->setattr($fd_stdin, TCSANOW); } sub getone { my $key = ''; cbreak(); sysread(STDIN, $key, 1); cooked(); return $key; } } END { cooked() } The Term::ReadKey module from CPAN may be easier to use. Recent versions include also support for non-portable systems as well. use Term::ReadKey; open my $tty, '<', '/dev/tty'; print "Gimme a char: "; ReadMode "raw"; my $key = ReadKey 0, $tty; ReadMode "normal"; printf "\nYou said %s, char number %03d\n", $key, ord $key; How can I tell whether there's a character waiting on a filehandle? The very first thing you should do is look into getting the Term::ReadKey extension from CPAN. As we mentioned earlier, it now even has limited support for non-portable (read: not open systems, closed, proprietary, not POSIX, not Unix, etc.) systems. You should also check out the Frequently Asked Questions list in comp.unix.* for things like this: the answer is essentially the same. It's very system-dependent. Here's one solution that works on BSD systems: sub key_ready { my($rin, $nfd); vec($rin, fileno(STDIN), 1) = 1; return $nfd = select($rin,undef,undef,0); } If you want to find out how many characters are waiting, there's also the FIONREAD ioctl call to be looked at. The *h2ph* tool that comes with Perl tries to convert C include files to Perl code, which can be "require"d. FIONREAD ends up defined as a function in the *sys/ioctl.ph* file: require './sys/ioctl.ph'; $size = pack("L", 0); ioctl(FH, FIONREAD(), $size) or die "Couldn't call ioctl: $!\n"; $size = unpack("L", $size); If *h2ph* wasn't installed or doesn't work for you, you can *grep* the include files by hand: % grep FIONREAD /usr/include/*/* /usr/include/asm/ioctls.h:#define FIONREAD 0x541B Or write a small C program using the editor of champions: % cat > fionread.c #include <sys/ioctl.h> main() { printf("%#08x\n", FIONREAD); } ^D % cc -o fionread fionread.c % ./fionread 0x4004667f And then hard-code it, leaving porting as an exercise to your successor. $FIONREAD = 0x4004667f; # XXX: opsys dependent $size = pack("L", 0); ioctl(FH, $FIONREAD, $size) or die "Couldn't call ioctl: $!\n"; $size = unpack("L", $size); FIONREAD requires a filehandle connected to a stream, meaning that sockets, pipes, and tty devices work, but *not* files. How do I do a "tail -f" in perl? First try seek($gw_fh, 0, 1); The statement "seek($gw_fh, 0, 1)" doesn't change the current position, but it does clear the end-of-file condition on the handle, so that the next "<$gw_fh>" makes Perl try again to read something. If that doesn't work (it relies on features of your stdio implementation), then you need something more like this: for (;;) { for ($curpos = tell($gw_fh); <$gw_fh>; $curpos =tell($gw_fh)) { # search for some stuff and put it into files } # sleep for a while seek($gw_fh, $curpos, 0); # seek to where we had been } If this still doesn't work, look into the "clearerr" method from IO::Handle, which resets the error and end-of-file states on the handle. There's also a File::Tail module from CPAN. How do I dup() a filehandle in Perl? If you check "open" in perlfunc, you'll see that several of the ways to call open() should do the trick. For example: open my $log, '>>', '/foo/logfile'; open STDERR, '>&', $log; Or even with a literal numeric descriptor: my $fd = $ENV{MHCONTEXTFD}; open $mhcontext, "<&=$fd"; # like fdopen(3S) Note that "<&STDIN" makes a copy, but "<&=STDIN" makes an alias. That means if you close an aliased handle, all aliases become inaccessible. This is not true with a copied one. Error checking, as always, has been left as an exercise for the reader. How do I close a file descriptor by number? If, for some reason, you have a file descriptor instead of a filehandle (perhaps you used "POSIX::open"), you can use the "close()" function from the POSIX module: use POSIX (); POSIX::close( $fd ); This should rarely be necessary, as the Perl "close()" function is to be used for things that Perl opened itself, even if it was a dup of a numeric descriptor as with "MHCONTEXT" above. But if you really have to, you may be able to do this: require './sys/syscall.ph'; my $rc = syscall(SYS_close(), $fd + 0); # must force numeric die "can't sysclose $fd: $!" unless $rc == -1; Or, just use the fdopen(3S) feature of "open()": { open my $fh, "<&=$fd" or die "Cannot reopen fd=$fd: $!"; close $fh; } Why doesn't glob("*.*") get all the files? Because even on non-Unix ports, Perl's glob function follows standard Unix globbing semantics. You'll need "glob("*")" to get all (non-hidden) files. This makes glob() portable even to legacy systems. Your port may include proprietary globbing functions as well. Check its documentation for details. Why does Perl let me delete read-only files? Why does "-i" clobber protected files? Isn't this a bug in Perl? This is elaborately and painstakingly described in the file-dir-perms article in the "Far More Than You Ever Wanted To Know" collection in <http://www.cpan.org/misc/olddoc/FMTEYEWTK.tgz> . The executive summary: learn how your filesystem works. The permissions on a file say what can happen to the data in that file. The permissions on a directory say what can happen to the list of files in that directory. If you delete a file, you're removing its name from the directory (so the operation depends on the permissions of the directory, not of the file). If you try to write to the file, the permissions of the file govern whether you're allowed to. How do I select a random line from a file? Short of loading the file into a database or pre-indexing the lines in the file, there are a couple of things that you can do. Here's a reservoir-sampling algorithm from the Camel Book: srand; rand($.) < 1 && ($line = $_) while <>; This has a significant advantage in space over reading the whole file in. You can find a proof of this method in *The Art of Computer Programming*, Volume 2, Section 3.4.2, by Donald E. Knuth. You can use the File::Random module which provides a function for that algorithm: use File::Random qw/random_line/; my $line = random_line($filename); Another way is to use the Tie::File module, which treats the entire file as an array. Simply access a random array element. Why do I get weird spaces when I print an array of lines? (contributed by brian d foy) If you are seeing spaces between the elements of your array when you print the array, you are probably interpolating the array in double quotes: my @animals = qw(camel llama alpaca vicuna); print "animals are: @animals\n"; It's the double quotes, not the "print", doing this. Whenever you interpolate an array in a double quote context, Perl joins the elements with spaces (or whatever is in $", which is a space by default): animals are: camel llama alpaca vicuna This is different than printing the array without the interpolation: my @animals = qw(camel llama alpaca vicuna); print "animals are: ", @animals, "\n"; Now the output doesn't have the spaces between the elements because the elements of @animals simply become part of the list to "print": animals are: camelllamaalpacavicuna You might notice this when each of the elements of @array end with a newline. You expect to print one element per line, but notice that every line after the first is indented: this is a line this is another line this is the third line That extra space comes from the interpolation of the array. If you don't want to put anything between your array elements, don't use the array in double quotes. You can send it to print without them: print @lines; How do I delete a directory tree? (contributed by brian d foy) If you have an empty directory, you can use Perl's built-in "rmdir". If the directory is not empty (so, with files or subdirectories), you either have to empty it yourself (a lot of work) or use a module to help you. The File::Path module, which comes with Perl, has a "remove_tree" which can take care of all of the hard work for you: use File::Path qw(remove_tree); remove_tree( @directories ); The File::Path module also has a legacy interface to the older "rmtree" subroutine. Found in /usr/share/perl/5.34/pod/perlfaq6.pod How can I hope to use regular expressions without creating illegible and unmaintainable code? Three techniques can make regular expressions maintainable and understandable. Comments Outside the Regex Describe what you're doing and how you're doing it, using normal Perl comments. # turn the line into the first word, a colon, and the # number of characters on the rest of the line s/^(\w+)(.*)/ lc($1) . ":" . length($2) /meg; Comments Inside the Regex The "/x" modifier causes whitespace to be ignored in a regex pattern (except in a character class and a few other places), and also allows you to use normal comments there, too. As you can imagine, whitespace and comments help a lot. "/x" lets you turn this: s{<(?:[^>'"]*|".*?"|'.*?')+>}{}gs; into this: s{ < # opening angle bracket (?: # Non-backreffing grouping paren [^>'"] * # 0 or more things that are neither > nor ' nor " | # or else ".*?" # a section between double quotes (stingy match) | # or else '.*?' # a section between single quotes (stingy match) ) + # all occurring one or more times > # closing angle bracket }{}gsx; # replace with nothing, i.e. delete It's still not quite so clear as prose, but it is very useful for describing the meaning of each part of the pattern. Different Delimiters While we normally think of patterns as being delimited with "/" characters, they can be delimited by almost any character. perlre describes this. For example, the "s///" above uses braces as delimiters. Selecting another delimiter can avoid quoting the delimiter within the pattern: s/\/usr\/local/\/usr\/share/g; # bad delimiter choice s#/usr/local#/usr/share#g; # better Using logically paired delimiters can be even more readable: s{/usr/local/}{/usr/share}g; # better still I'm having trouble matching over more than one line. What's wrong? Either you don't have more than one line in the string you're looking at (probably), or else you aren't using the correct modifier(s) on your pattern (possibly). There are many ways to get multiline data into a string. If you want it to happen automatically while reading input, you'll want to set $/ (probably to '' for paragraphs or "undef" for the whole file) to allow you to read more than one line at a time. Read perlre to help you decide which of "/s" and "/m" (or both) you might want to use: "/s" allows dot to include newline, and "/m" allows caret and dollar to match next to a newline, not just at the end of the string. You do need to make sure that you've actually got a multiline string in there. For example, this program detects duplicate words, even when they span line breaks (but not paragraph ones). For this example, we don't need "/s" because we aren't using dot in a regular expression that we want to cross line boundaries. Neither do we need "/m" because we don't want caret or dollar to match at any point inside the record next to newlines. But it's imperative that $/ be set to something other than the default, or else we won't actually ever have a multiline record read in. $/ = ''; # read in whole paragraph, not just one line while ( <> ) { while ( /\b([\w'-]+)(\s+\g1)+\b/gi ) { # word starts alpha print "Duplicate $1 at paragraph $.\n"; } } Here's some code that finds sentences that begin with "From " (which would be mangled by many mailers): $/ = ''; # read in whole paragraph, not just one line while ( <> ) { while ( /^From /gm ) { # /m makes ^ match next to \n print "leading From in paragraph $.\n"; } } Here's code that finds everything between START and END in a paragraph: undef $/; # read in whole file, not just one line or paragraph while ( <> ) { while ( /START(.*?)END/sgm ) { # /s makes . cross line boundaries print "$1\n"; } } How can I pull out lines between two patterns that are themselves on different lines? You can use Perl's somewhat exotic ".." operator (documented in perlop): perl -ne 'print if /START/ .. /END/' file1 file2 ... If you wanted text and not lines, you would use perl -0777 -ne 'print "$1\n" while /START(.*?)END/gs' file1 file2 ... But if you want nested occurrences of "START" through "END", you'll run up against the problem described in the question in this section on matching balanced text. Here's another example of using "..": while (<>) { my $in_header = 1 .. /^$/; my $in_body = /^$/ .. eof; # now choose between them } continue { $. = 0 if eof; # fix $. } How do I match XML, HTML, or other nasty, ugly things with a regex? Do not use regexes. Use a module and forget about the regular expressions. The XML::LibXML, HTML::TokeParser and HTML::TreeBuilder modules are good starts, although each namespace has other parsing modules specialized for certain tasks and different ways of doing it. Start at CPAN Search ( <http://metacpan.org/> ) and wonder at all the work people have done for you already! :) I put a regular expression into $/ but it didn't work. What's wrong? $/ has to be a string. You can use these examples if you really need to do this. If you have File::Stream, this is easy. use File::Stream; my $stream = File::Stream->new( $filehandle, separator => qr/\s*,\s*/, ); print "$_\n" while <$stream>; If you don't have File::Stream, you have to do a little more work. You can use the four-argument form of sysread to continually add to a buffer. After you add to the buffer, you check if you have a complete line (using your regular expression). local $_ = ""; while( sysread FH, $_, 8192, length ) { while( s/^((?s).*?)your_pattern// ) { my $record = $1; # do stuff here. } } You can do the same thing with foreach and a match using the c flag and the \G anchor, if you do not mind your entire file being in memory at the end. local $_ = ""; while( sysread FH, $_, 8192, length ) { foreach my $record ( m/\G((?s).*?)your_pattern/gc ) { # do stuff here. } substr( $_, 0, pos ) = "" if pos; } How do I substitute case-insensitively on the LHS while preserving case on the RHS? Here's a lovely Perlish solution by Larry Rosler. It exploits properties of bitwise xor on ASCII strings. $_= "this is a TEsT case"; $old = 'test'; $new = 'success'; s{(\Q$old\E)} { uc $new | (uc $1 ^ $1) . (uc(substr $1, -1) ^ substr $1, -1) x (length($new) - length $1) }egi; print; And here it is as a subroutine, modeled after the above: sub preserve_case { my ($old, $new) = @_; my $mask = uc $old ^ $old; uc $new | $mask . substr($mask, -1) x (length($new) - length($old)) } $string = "this is a TEsT case"; $string =~ s/(test)/preserve_case($1, "success")/egi; print "$string\n"; This prints: this is a SUcCESS case As an alternative, to keep the case of the replacement word if it is longer than the original, you can use this code, by Jeff Pinyan: sub preserve_case { my ($from, $to) = @_; my ($lf, $lt) = map length, @_; if ($lt < $lf) { $from = substr $from, 0, $lt } else { $from .= substr $to, $lf } return uc $to | ($from ^ uc $from); } This changes the sentence to "this is a SUcCess case." Just to show that C programmers can write C in any programming language, if you prefer a more C-like solution, the following script makes the substitution have the same case, letter by letter, as the original. (It also happens to run about 240% slower than the Perlish solution runs.) If the substitution has more characters than the string being substituted, the case of the last character is used for the rest of the substitution. # Original by Nathan Torkington, massaged by Jeffrey Friedl # sub preserve_case { my ($old, $new) = @_; my $state = 0; # 0 = no change; 1 = lc; 2 = uc my ($i, $oldlen, $newlen, $c) = (0, length($old), length($new)); my $len = $oldlen < $newlen ? $oldlen : $newlen; for ($i = 0; $i < $len; $i++) { if ($c = substr($old, $i, 1), $c =~ /[\W\d_]/) { $state = 0; } elsif (lc $c eq $c) { substr($new, $i, 1) = lc(substr($new, $i, 1)); $state = 1; } else { substr($new, $i, 1) = uc(substr($new, $i, 1)); $state = 2; } } # finish up with any remaining new (for when new is longer than old) if ($newlen > $oldlen) { if ($state == 1) { substr($new, $oldlen) = lc(substr($new, $oldlen)); } elsif ($state == 2) { substr($new, $oldlen) = uc(substr($new, $oldlen)); } } return $new; } How can I make "\w" match national character sets? Put "use locale;" in your script. The \w character class is taken from the current locale. See perllocale for details. How can I match a locale-smart version of "/[a-zA-Z]/"? You can use the POSIX character class syntax "/[[:alpha:]]/" documented in perlre. No matter which locale you are in, the alphabetic characters are the characters in \w without the digits and the underscore. As a regex, that looks like "/[^\W\d_]/". Its complement, the non-alphabetics, is then everything in \W along with the digits and the underscore, or "/[\W\d_]/". How can I quote a variable to use in a regex? The Perl parser will expand $variable and @variable references in regular expressions unless the delimiter is a single quote. Remember, too, that the right-hand side of a "s///" substitution is considered a double-quoted string (see perlop for more details). Remember also that any regex special characters will be acted on unless you precede the substitution with \Q. Here's an example: $string = "Placido P. Octopus"; $regex = "P."; $string =~ s/$regex/Polyp/; # $string is now "Polypacido P. Octopus" Because "." is special in regular expressions, and can match any single character, the regex "P." here has matched the <Pl> in the original string. To escape the special meaning of ".", we use "\Q": $string = "Placido P. Octopus"; $regex = "P."; $string =~ s/\Q$regex/Polyp/; # $string is now "Placido Polyp Octopus" The use of "\Q" causes the "." in the regex to be treated as a regular character, so that "P." matches a "P" followed by a dot. What is "/o" really for? (contributed by brian d foy) The "/o" option for regular expressions (documented in perlop and perlreref) tells Perl to compile the regular expression only once. This is only useful when the pattern contains a variable. Perls 5.6 and later handle this automatically if the pattern does not change. Since the match operator "m//", the substitution operator "s///", and the regular expression quoting operator "qr//" are double-quotish constructs, you can interpolate variables into the pattern. See the answer to "How can I quote a variable to use in a regex?" for more details. This example takes a regular expression from the argument list and prints the lines of input that match it: my $pattern = shift @ARGV; while( <> ) { print if m/$pattern/; } Versions of Perl prior to 5.6 would recompile the regular expression for each iteration, even if $pattern had not changed. The "/o" would prevent this by telling Perl to compile the pattern the first time, then reuse that for subsequent iterations: my $pattern = shift @ARGV; while( <> ) { print if m/$pattern/o; # useful for Perl < 5.6 } In versions 5.6 and later, Perl won't recompile the regular expression if the variable hasn't changed, so you probably don't need the "/o" option. It doesn't hurt, but it doesn't help either. If you want any version of Perl to compile the regular expression only once even if the variable changes (thus, only using its initial value), you still need the "/o". You can watch Perl's regular expression engine at work to verify for yourself if Perl is recompiling a regular expression. The "use re 'debug'" pragma (comes with Perl 5.005 and later) shows the details. With Perls before 5.6, you should see "re" reporting that its compiling the regular expression on each iteration. With Perl 5.6 or later, you should only see "re" report that for the first iteration. use re 'debug'; my $regex = 'Perl'; foreach ( qw(Perl Java Ruby Python) ) { print STDERR "-" x 73, "\n"; print STDERR "Trying $_...\n"; print STDERR "\t$_ is good!\n" if m/$regex/; } How do I use a regular expression to strip C-style comments from a file? While this actually can be done, it's much harder than you'd think. For example, this one-liner perl -0777 -pe 's{/\*.*?\*/}{}gs' foo.c will work in many but not all cases. You see, it's too simple-minded for certain kinds of C programs, in particular, those with what appear to be comments in quoted strings. For that, you'd need something like this, created by Jeffrey Friedl and later modified by Fred Curtis. $/ = undef; $_ = <>; s#/\*[^*]*\*+([^/*][^*]*\*+)*/|("(\\.|[^"\\])*"|'(\\.|[^'\\])*'|.[^/"'\\]*)#defined $2 ? $2 : ""#gse; print; This could, of course, be more legibly written with the "/x" modifier, adding whitespace and comments. Here it is expanded, courtesy of Fred Curtis. s{ /\* ## Start of /* ... */ comment [^*]*\*+ ## Non-* followed by 1-or-more *'s ( [^/*][^*]*\*+ )* ## 0-or-more things which don't start with / ## but do end with '*' / ## End of /* ... */ comment | ## OR various things which aren't comments: ( " ## Start of " ... " string ( \\. ## Escaped char | ## OR [^"\\] ## Non "\ )* " ## End of " ... " string | ## OR ' ## Start of ' ... ' string ( \\. ## Escaped char | ## OR [^'\\] ## Non '\ )* ' ## End of ' ... ' string | ## OR . ## Anything other char [^/"'\\]* ## Chars which doesn't start a comment, string or escape ) }{defined $2 ? $2 : ""}gxse; A slight modification also removes C++ comments, possibly spanning multiple lines using a continuation character: s#/\*[^*]*\*+([^/*][^*]*\*+)*/|//([^\\]|[^\n][\n]?)*?\n|("(\\.|[^"\\])*"|'(\\.|[^'\\])*'|.[^/"'\\]*)#defined $3 ? $3 : ""#gse; Can I use Perl regular expressions to match balanced text? (contributed by brian d foy) Your first try should probably be the Text::Balanced module, which is in the Perl standard library since Perl 5.8. It has a variety of functions to deal with tricky text. The Regexp::Common module can also help by providing canned patterns you can use. As of Perl 5.10, you can match balanced text with regular expressions using recursive patterns. Before Perl 5.10, you had to resort to various tricks such as using Perl code in "(??{})" sequences. Here's an example using a recursive regular expression. The goal is to capture all of the text within angle brackets, including the text in nested angle brackets. This sample text has two "major" groups: a group with one level of nesting and a group with two levels of nesting. There are five total groups in angle brackets: I have some <brackets in <nested brackets> > and <another group <nested once <nested twice> > > and that's it. The regular expression to match the balanced text uses two new (to Perl 5.10) regular expression features. These are covered in perlre and this example is a modified version of one in that documentation. First, adding the new possessive "+" to any quantifier finds the longest match and does not backtrack. That's important since you want to handle any angle brackets through the recursion, not backtracking. The group "[^<>]++" finds one or more non-angle brackets without backtracking. Second, the new "(?PARNO)" refers to the sub-pattern in the particular capture group given by "PARNO". In the following regex, the first capture group finds (and remembers) the balanced text, and you need that same pattern within the first buffer to get past the nested text. That's the recursive part. The "(?1)" uses the pattern in the outer capture group as an independent part of the regex. Putting it all together, you have: #!/usr/local/bin/perl5.10.0 my $string =<<"HERE"; I have some <brackets in <nested brackets> > and <another group <nested once <nested twice> > > and that's it. HERE my @groups = $string =~ m/ ( # start of capture group 1 < # match an opening angle bracket (?: [^<>]++ # one or more non angle brackets, non backtracking | (?1) # found < or >, so recurse to capture group 1 )* > # match a closing angle bracket ) # end of capture group 1 /xg; $" = "\n\t"; print "Found:\n\t@groups\n"; The output shows that Perl found the two major groups: Found: <brackets in <nested brackets> > <another group <nested once <nested twice> > > With a little extra work, you can get all of the groups in angle brackets even if they are in other angle brackets too. Each time you get a balanced match, remove its outer delimiter (that's the one you just matched so don't match it again) and add it to a queue of strings to process. Keep doing that until you get no matches: #!/usr/local/bin/perl5.10.0 my @queue =<<"HERE"; I have some <brackets in <nested brackets> > and <another group <nested once <nested twice> > > and that's it. HERE my $regex = qr/ ( # start of bracket 1 < # match an opening angle bracket (?: [^<>]++ # one or more non angle brackets, non backtracking | (?1) # recurse to bracket 1 )* > # match a closing angle bracket ) # end of bracket 1 /x; $" = "\n\t"; while( @queue ) { my $string = shift @queue; my @groups = $string =~ m/$regex/g; print "Found:\n\t@groups\n\n" if @groups; unshift @queue, map { s/^<//; s/>$//; $_ } @groups; } The output shows all of the groups. The outermost matches show up first and the nested matches show up later: Found: <brackets in <nested brackets> > <another group <nested once <nested twice> > > Found: <nested brackets> Found: <nested once <nested twice> > Found: <nested twice> How do I process each word on each line? Use the split function: while (<>) { foreach my $word ( split ) { # do something with $word here } } Note that this isn't really a word in the English sense; it's just chunks of consecutive non-whitespace characters. To work with only alphanumeric sequences (including underscores), you might consider while (<>) { foreach $word (m/(\w+)/g) { # do something with $word here } } How can I print out a word-frequency or line-frequency summary? To do this, you have to parse out each word in the input stream. We'll pretend that by word you mean chunk of alphabetics, hyphens, or apostrophes, rather than the non-whitespace chunk idea of a word given in the previous question: my (%seen); while (<>) { while ( /(\b[^\W_\d][\w'-]+\b)/g ) { # misses "`sheep'" $seen{$1}++; } } while ( my ($word, $count) = each %seen ) { print "$count $word\n"; } If you wanted to do the same thing for lines, you wouldn't need a regular expression: my (%seen); while (<>) { $seen{$_}++; } while ( my ($line, $count) = each %seen ) { print "$count $line"; } If you want these output in a sorted order, see perlfaq4: "How do I sort a hash (optionally by value instead of key)?". How do I efficiently match many regular expressions at once? (contributed by brian d foy) You want to avoid compiling a regular expression every time you want to match it. In this example, perl must recompile the regular expression for every iteration of the "foreach" loop since $pattern can change: my @patterns = qw( fo+ ba[rz] ); LINE: while( my $line = <> ) { foreach my $pattern ( @patterns ) { if( $line =~ m/\b$pattern\b/i ) { print $line; next LINE; } } } The "qr//" operator compiles a regular expression, but doesn't apply it. When you use the pre-compiled version of the regex, perl does less work. In this example, I inserted a "map" to turn each pattern into its pre-compiled form. The rest of the script is the same, but faster: my @patterns = map { qr/\b$_\b/i } qw( fo+ ba[rz] ); LINE: while( my $line = <> ) { foreach my $pattern ( @patterns ) { if( $line =~ m/$pattern/ ) { print $line; next LINE; } } } In some cases, you may be able to make several patterns into a single regular expression. Beware of situations that require backtracking though. In this example, the regex is only compiled once because $regex doesn't change between iterations: my $regex = join '|', qw( fo+ ba[rz] ); while( my $line = <> ) { print if $line =~ m/\b(?:$regex)\b/i; } The function "list2re" in Data::Munge on CPAN can also be used to form a single regex that matches a list of literal strings (not regexes). For more details on regular expression efficiency, see *Mastering Regular Expressions* by Jeffrey Friedl. He explains how the regular expressions engine works and why some patterns are surprisingly inefficient. Once you understand how perl applies regular expressions, you can tune them for individual situations. Why does using $&, $`, or $' slow my program down? (contributed by Anno Siegel) Once Perl sees that you need one of these variables anywhere in the program, it provides them on each and every pattern match. That means that on every pattern match the entire string will be copied, part of it to $`, part to $&, and part to $'. Thus the penalty is most severe with long strings and patterns that match often. Avoid $&, $', and $` if you can, but if you can't, once you've used them at all, use them at will because you've already paid the price. Remember that some algorithms really appreciate them. As of the 5.005 release, the $& variable is no longer "expensive" the way the other two are. Since Perl 5.6.1 the special variables @- and @+ can functionally replace $`, $& and $'. These arrays contain pointers to the beginning and end of each match (see perlvar for the full story), so they give you essentially the same information, but without the risk of excessive string copying. Perl 5.10 added three specials, "${^MATCH}", "${^PREMATCH}", and "${^POSTMATCH}" to do the same job but without the global performance penalty. Perl 5.10 only sets these variables if you compile or execute the regular expression with the "/p" modifier. What good is "\G" in a regular expression? You use the "\G" anchor to start the next match on the same string where the last match left off. The regular expression engine cannot skip over any characters to find the next match with this anchor, so "\G" is similar to the beginning of string anchor, "^". The "\G" anchor is typically used with the "g" modifier. It uses the value of "pos()" as the position to start the next match. As the match operator makes successive matches, it updates "pos()" with the position of the next character past the last match (or the first character of the next match, depending on how you like to look at it). Each string has its own "pos()" value. Suppose you want to match all of consecutive pairs of digits in a string like "1122a44" and stop matching when you encounter non-digits. You want to match 11 and 22 but the letter "a" shows up between 22 and 44 and you want to stop at "a". Simply matching pairs of digits skips over the "a" and still matches 44. $_ = "1122a44"; my @pairs = m/(\d\d)/g; # qw( 11 22 44 ) If you use the "\G" anchor, you force the match after 22 to start with the "a". The regular expression cannot match there since it does not find a digit, so the next match fails and the match operator returns the pairs it already found. $_ = "1122a44"; my @pairs = m/\G(\d\d)/g; # qw( 11 22 ) You can also use the "\G" anchor in scalar context. You still need the "g" modifier. $_ = "1122a44"; while( m/\G(\d\d)/g ) { print "Found $1\n"; } After the match fails at the letter "a", perl resets "pos()" and the next match on the same string starts at the beginning. $_ = "1122a44"; while( m/\G(\d\d)/g ) { print "Found $1\n"; } print "Found $1 after while" if m/(\d\d)/g; # finds "11" You can disable "pos()" resets on fail with the "c" modifier, documented in perlop and perlreref. Subsequent matches start where the last successful match ended (the value of "pos()") even if a match on the same string has failed in the meantime. In this case, the match after the "while()" loop starts at the "a" (where the last match stopped), and since it does not use any anchor it can skip over the "a" to find 44. $_ = "1122a44"; while( m/\G(\d\d)/gc ) { print "Found $1\n"; } print "Found $1 after while" if m/(\d\d)/g; # finds "44" Typically you use the "\G" anchor with the "c" modifier when you want to try a different match if one fails, such as in a tokenizer. Jeffrey Friedl offers this example which works in 5.004 or later. while (<>) { chomp; PARSER: { m/ \G( \d+\b )/gcx && do { print "number: $1\n"; redo; }; m/ \G( \w+ )/gcx && do { print "word: $1\n"; redo; }; m/ \G( \s+ )/gcx && do { print "space: $1\n"; redo; }; m/ \G( [^\w\d]+ )/gcx && do { print "other: $1\n"; redo; }; } } For each line, the "PARSER" loop first tries to match a series of digits followed by a word boundary. This match has to start at the place the last match left off (or the beginning of the string on the first match). Since "m/ \G( \d+\b )/gcx" uses the "c" modifier, if the string does not match that regular expression, perl does not reset pos() and the next match starts at the same position to try a different pattern. Are Perl regexes DFAs or NFAs? Are they POSIX compliant? While it's true that Perl's regular expressions resemble the DFAs (deterministic finite automata) of the egrep(1) program, they are in fact implemented as NFAs (non-deterministic finite automata) to allow backtracking and backreferencing. And they aren't POSIX-style either, because those guarantee worst-case behavior for all cases. (It seems that some people prefer guarantees of consistency, even when what's guaranteed is slowness.) See the book "Mastering Regular Expressions" (from O'Reilly) by Jeffrey Friedl for all the details you could ever hope to know on these matters (a full citation appears in perlfaq2). How can I match strings with multibyte characters? Starting from Perl 5.6 Perl has had some level of multibyte character support. Perl 5.8 or later is recommended. Supported multibyte character repertoires include Unicode, and legacy encodings through the Encode module. See perluniintro, perlunicode, and Encode. If you are stuck with older Perls, you can do Unicode with the Unicode::String module, and character conversions using the Unicode::Map8 and Unicode::Map modules. If you are using Japanese encodings, you might try using the jperl 5.005_03. Finally, the following set of approaches was offered by Jeffrey Friedl, whose article in issue #5 of The Perl Journal talks about this very matter. Let's suppose you have some weird Martian encoding where pairs of ASCII uppercase letters encode single Martian letters (i.e. the two bytes "CV" make a single Martian letter, as do the two bytes "SG", "VS", "XX", etc.). Other bytes represent single characters, just like ASCII. So, the string of Martian "I am CVSGXX!" uses 12 bytes to encode the nine characters 'I', ' ', 'a', 'm', ' ', 'CV', 'SG', 'XX', '!'. Now, say you want to search for the single character "/GX/". Perl doesn't know about Martian, so it'll find the two bytes "GX" in the "I am CVSGXX!" string, even though that character isn't there: it just looks like it is because "SG" is next to "XX", but there's no real "GX". This is a big problem. Here are a few ways, all painful, to deal with it: # Make sure adjacent "martian" bytes are no longer adjacent. $martian =~ s/([A-Z][A-Z])/ $1 /g; print "found GX!\n" if $martian =~ /GX/; Or like this: my @chars = $martian =~ m/([A-Z][A-Z]|[^A-Z])/g; # above is conceptually similar to: my @chars = $text =~ m/(.)/g; # foreach my $char (@chars) { print "found GX!\n", last if $char eq 'GX'; } Or like this: while ($martian =~ m/\G([A-Z][A-Z]|.)/gs) { # \G probably unneeded if ($1 eq 'GX') { print "found GX!\n"; last; } } Here's another, slightly less painful, way to do it from Benjamin Goldberg, who uses a zero-width negative look-behind assertion. print "found GX!\n" if $martian =~ m/ (?<![A-Z]) (?:[A-Z][A-Z])*? GX /x; This succeeds if the "martian" character GX is in the string, and fails otherwise. If you don't like using (?<!), a zero-width negative look-behind assertion, you can replace (?<![A-Z]) with (?:^|[^A-Z]). It does have the drawback of putting the wrong thing in $-[0] and $+[0], but this usually can be worked around. How do I match a regular expression that's in a variable? (contributed by brian d foy) We don't have to hard-code patterns into the match operator (or anything else that works with regular expressions). We can put the pattern in a variable for later use. The match operator is a double quote context, so you can interpolate your variable just like a double quoted string. In this case, you read the regular expression as user input and store it in $regex. Once you have the pattern in $regex, you use that variable in the match operator. chomp( my $regex = <STDIN> ); if( $string =~ m/$regex/ ) { ... } Any regular expression special characters in $regex are still special, and the pattern still has to be valid or Perl will complain. For instance, in this pattern there is an unpaired parenthesis. my $regex = "Unmatched ( paren"; "Two parens to bind them all" =~ m/$regex/; When Perl compiles the regular expression, it treats the parenthesis as the start of a memory match. When it doesn't find the closing parenthesis, it complains: Unmatched ( in regex; marked by <-- HERE in m/Unmatched ( <-- HERE paren/ at script line 3. You can get around this in several ways depending on our situation. First, if you don't want any of the characters in the string to be special, you can escape them with "quotemeta" before you use the string. chomp( my $regex = <STDIN> ); $regex = quotemeta( $regex ); if( $string =~ m/$regex/ ) { ... } You can also do this directly in the match operator using the "\Q" and "\E" sequences. The "\Q" tells Perl where to start escaping special characters, and the "\E" tells it where to stop (see perlop for more details). chomp( my $regex = <STDIN> ); if( $string =~ m/\Q$regex\E/ ) { ... } Alternately, you can use "qr//", the regular expression quote operator (see perlop for more details). It quotes and perhaps compiles the pattern, and you can apply regular expression flags to the pattern. chomp( my $input = <STDIN> ); my $regex = qr/$input/is; $string =~ m/$regex/ # same as m/$input/is; You might also want to trap any errors by wrapping an "eval" block around the whole thing. chomp( my $input = <STDIN> ); eval { if( $string =~ m/\Q$input\E/ ) { ... } }; warn $@ if $@; Or... my $regex = eval { qr/$input/is }; if( defined $regex ) { $string =~ m/$regex/; } else { warn $@; } Found in /usr/share/perl/5.34/pod/perlfaq7.pod Can I get a BNF/yacc/RE for the Perl language? There is no BNF, but you can paw your way through the yacc grammar in perly.y in the source distribution if you're particularly brave. The grammar relies on very smart tokenizing code, so be prepared to venture into toke.c as well. In the words of Chaim Frenkel: "Perl's grammar can not be reduced to BNF. The work of parsing perl is distributed between yacc, the lexer, smoke and mirrors." What are all these $@%&* punctuation signs, and how do I know when to use them? They are type specifiers, as detailed in perldata: $ for scalar values (number, string or reference) @ for arrays % for hashes (associative arrays) & for subroutines (aka functions, procedures, methods) * for all types of that symbol name. In version 4 you used them like pointers, but in modern perls you can just use references. There are a couple of other symbols that you're likely to encounter that aren't really type specifiers: <> are used for inputting a record from a filehandle. \ takes a reference to something. Note that <FILE> is *neither* the type specifier for files nor the name of the handle. It is the "<>" operator applied to the handle FILE. It reads one line (well, record--see "$/" in perlvar) from the handle FILE in scalar context, or *all* lines in list context. When performing open, close, or any other operation besides "<>" on files, or even when talking about the handle, do *not* use the brackets. These are correct: "eof(FH)", "seek(FH, 0, 2)" and "copying from STDIN to FILE". Do I always/never have to quote my strings or use semicolons and commas? Normally, a bareword doesn't need to be quoted, but in most cases probably should be (and must be under "use strict"). But a hash key consisting of a simple word and the left-hand operand to the "=>" operator both count as though they were quoted: This is like this ------------ --------------- $foo{line} $foo{'line'} bar => stuff 'bar' => stuff The final semicolon in a block is optional, as is the final comma in a list. Good style (see perlstyle) says to put them in except for one-liners: if ($whoops) { exit 1 } my @nums = (1, 2, 3); if ($whoops) { exit 1; } my @lines = ( "There Beren came from mountains cold", "And lost he wandered under leaves", ); How do I skip some return values? One way is to treat the return values as a list and index into it: $dir = (getpwnam($user))[7]; Another way is to use undef as an element on the left-hand-side: ($dev, $ino, undef, undef, $uid, $gid) = stat($file); You can also use a list slice to select only the elements that you need: ($dev, $ino, $uid, $gid) = ( stat($file) )[0,1,4,5]; How do I temporarily block warnings? If you are running Perl 5.6.0 or better, the "use warnings" pragma allows fine control of what warnings are produced. See perllexwarn for more details. { no warnings; # temporarily turn off warnings $x = $y + $z; # I know these might be undef } Additionally, you can enable and disable categories of warnings. You turn off the categories you want to ignore and you can still get other categories of warnings. See perllexwarn for the complete details, including the category names and hierarchy. { no warnings 'uninitialized'; $x = $y + $z; } If you have an older version of Perl, the $^W variable (documented in perlvar) controls runtime warnings for a block: { local $^W = 0; # temporarily turn off warnings $x = $y + $z; # I know these might be undef } Note that like all the punctuation variables, you cannot currently use my() on $^W, only local(). Why do Perl operators have different precedence than C operators? Actually, they don't. All C operators that Perl copies have the same precedence in Perl as they do in C. The problem is with operators that C doesn't have, especially functions that give a list context to everything on their right, eg. print, chmod, exec, and so on. Such functions are called "list operators" and appear as such in the precedence table in perlop. A common mistake is to write: unlink $file || die "snafu"; This gets interpreted as: unlink ($file || die "snafu"); To avoid this problem, either put in extra parentheses or use the super low precedence "or" operator: (unlink $file) || die "snafu"; unlink $file or die "snafu"; The "English" operators ("and", "or", "xor", and "not") deliberately have precedence lower than that of list operators for just such situations as the one above. Another operator with surprising precedence is exponentiation. It binds more tightly even than unary minus, making "-2**2" produce a negative four and not a positive one. It is also right-associating, meaning that "2**3**2" is two raised to the ninth power, not eight squared. Although it has the same precedence as in C, Perl's "?:" operator produces an lvalue. This assigns $x to either $if_true or $if_false, depending on the trueness of $maybe: ($maybe ? $if_true : $if_false) = $x; How do I declare/create a structure? In general, you don't "declare" a structure. Just use a (probably anonymous) hash reference. See perlref and perldsc for details. Here's an example: $person = {}; # new anonymous hash $person->{AGE} = 24; # set field AGE to 24 $person->{NAME} = "Nat"; # set field NAME to "Nat" If you're looking for something a bit more rigorous, try perlootut. How do I create a module? perlnewmod is a good place to start, ignore the bits about uploading to CPAN if you don't want to make your module publicly available. ExtUtils::ModuleMaker and Module::Starter are also good places to start. Many CPAN authors now use Dist::Zilla to automate as much as possible. Detailed documentation about modules can be found at: perlmod, perlmodlib, perlmodstyle. If you need to include C code or C library interfaces use h2xs. h2xs will create the module distribution structure and the initial interface files. perlxs and perlxstut explain the details. How do I adopt or take over a module already on CPAN? Ask the current maintainer to make you a co-maintainer or transfer the module to you. If you can not reach the author for some reason contact the PAUSE admins at modules AT perl.org who may be able to help, but each case is treated separately. * Get a login for the Perl Authors Upload Server (PAUSE) if you don't already have one: <http://pause.perl.org> * Write to modules AT perl.org explaining what you did to contact the current maintainer. The PAUSE admins will also try to reach the maintainer. * Post a public message in a heavily trafficked site announcing your intention to take over the module. * Wait a bit. The PAUSE admins don't want to act too quickly in case the current maintainer is on holiday. If there's no response to private communication or the public post, a PAUSE admin can transfer it to you. How do I create a class? (contributed by brian d foy) In Perl, a class is just a package, and methods are just subroutines. Perl doesn't get more formal than that and lets you set up the package just the way that you like it (that is, it doesn't set up anything for you). See also perlootut, a tutorial that covers class creation, and perlobj. How can I tell if a variable is tainted? You can use the tainted() function of the Scalar::Util module, available from CPAN (or included with Perl since release 5.8.0). See also "Laundering and Detecting Tainted Data" in perlsec. What's a closure? Closures are documented in perlref. *Closure* is a computer science term with a precise but hard-to-explain meaning. Usually, closures are implemented in Perl as anonymous subroutines with lasting references to lexical variables outside their own scopes. These lexicals magically refer to the variables that were around when the subroutine was defined (deep binding). Closures are most often used in programming languages where you can have the return value of a function be itself a function, as you can in Perl. Note that some languages provide anonymous functions but are not capable of providing proper closures: the Python language, for example. For more information on closures, check out any textbook on functional programming. Scheme is a language that not only supports but encourages closures. Here's a classic non-closure function-generating function: sub add_function_generator { return sub { shift() + shift() }; } my $add_sub = add_function_generator(); my $sum = $add_sub->(4,5); # $sum is 9 now. The anonymous subroutine returned by add_function_generator() isn't technically a closure because it refers to no lexicals outside its own scope. Using a closure gives you a *function template* with some customization slots left out to be filled later. Contrast this with the following make_adder() function, in which the returned anonymous function contains a reference to a lexical variable outside the scope of that function itself. Such a reference requires that Perl return a proper closure, thus locking in for all time the value that the lexical had when the function was created. sub make_adder { my $addpiece = shift; return sub { shift() + $addpiece }; } my $f1 = make_adder(20); my $f2 = make_adder(555); Now "$f1->($n)" is always 20 plus whatever $n you pass in, whereas "$f2->($n)" is always 555 plus whatever $n you pass in. The $addpiece in the closure sticks around. Closures are often used for less esoteric purposes. For example, when you want to pass in a bit of code into a function: my $line; timeout( 30, sub { $line = <STDIN> } ); If the code to execute had been passed in as a string, '$line = <STDIN>', there would have been no way for the hypothetical timeout() function to access the lexical variable $line back in its caller's scope. Another use for a closure is to make a variable *private* to a named subroutine, e.g. a counter that gets initialized at creation time of the sub and can only be modified from within the sub. This is sometimes used with a BEGIN block in package files to make sure a variable doesn't get meddled with during the lifetime of the package: BEGIN { my $id = 0; sub next_id { ++$id } } This is discussed in more detail in perlsub; see the entry on *Persistent Private Variables*. What is variable suicide and how can I prevent it? This problem was fixed in perl 5.004_05, so preventing it means upgrading your version of perl. ;) Variable suicide is when you (temporarily or permanently) lose the value of a variable. It is caused by scoping through my() and local() interacting with either closures or aliased foreach() iterator variables and subroutine arguments. It used to be easy to inadvertently lose a variable's value this way, but now it's much harder. Take this code: my $f = 'foo'; sub T { while ($i++ < 3) { my $f = $f; $f .= "bar"; print $f, "\n" } } T; print "Finally $f\n"; If you are experiencing variable suicide, that "my $f" in the subroutine doesn't pick up a fresh copy of the $f whose value is 'foo'. The output shows that inside the subroutine the value of $f leaks through when it shouldn't, as in this output: foobar foobarbar foobarbarbar Finally foo The $f that has "bar" added to it three times should be a new $f "my $f" should create a new lexical variable each time through the loop. The expected output is: foobar foobar foobar Finally foo How can I pass/return a {Function, FileHandle, Array, Hash, Method, Regex}? You need to pass references to these objects. See "Pass by Reference" in perlsub for this particular question, and perlref for information on references. Passing Variables and Functions Regular variables and functions are quite easy to pass: just pass in a reference to an existing or anonymous variable or function: func( \$some_scalar ); func( \@some_array ); func( [ 1 .. 10 ] ); func( \%some_hash ); func( { this => 10, that => 20 } ); func( \&some_func ); func( sub { $_[0] ** $_[1] } ); Passing Filehandles As of Perl 5.6, you can represent filehandles with scalar variables which you treat as any other scalar. open my $fh, $filename or die "Cannot open $filename! $!"; func( $fh ); sub func { my $passed_fh = shift; my $line = <$passed_fh>; } Before Perl 5.6, you had to use the *FH or "\*FH" notations. These are "typeglobs"--see "Typeglobs and Filehandles" in perldata and especially "Pass by Reference" in perlsub for more information. Passing Regexes Here's an example of how to pass in a string and a regular expression for it to match against. You construct the pattern with the "qr//" operator: sub compare { my ($val1, $regex) = @_; my $retval = $val1 =~ /$regex/; return $retval; } $match = compare("old McDonald", qr/d.*D/i); Passing Methods To pass an object method into a subroutine, you can do this: call_a_lot(10, $some_obj, "methname") sub call_a_lot { my ($count, $widget, $trick) = @_; for (my $i = 0; $i < $count; $i++) { $widget->$trick(); } } Or, you can use a closure to bundle up the object, its method call, and arguments: my $whatnot = sub { $some_obj->obfuscate(@args) }; func($whatnot); sub func { my $code = shift; &$code(); } You could also investigate the can() method in the UNIVERSAL class (part of the standard perl distribution). How do I create a static variable? (contributed by brian d foy) In Perl 5.10, declare the variable with "state". The "state" declaration creates the lexical variable that persists between calls to the subroutine: sub counter { state $count = 1; $count++ } You can fake a static variable by using a lexical variable which goes out of scope. In this example, you define the subroutine "counter", and it uses the lexical variable $count. Since you wrap this in a BEGIN block, $count is defined at compile-time, but also goes out of scope at the end of the BEGIN block. The BEGIN block also ensures that the subroutine and the value it uses is defined at compile-time so the subroutine is ready to use just like any other subroutine, and you can put this code in the same place as other subroutines in the program text (i.e. at the end of the code, typically). The subroutine "counter" still has a reference to the data, and is the only way you can access the value (and each time you do, you increment the value). The data in chunk of memory defined by $count is private to "counter". BEGIN { my $count = 1; sub counter { $count++ } } my $start = counter(); .... # code that calls counter(); my $end = counter(); In the previous example, you created a function-private variable because only one function remembered its reference. You could define multiple functions while the variable is in scope, and each function can share the "private" variable. It's not really "static" because you can access it outside the function while the lexical variable is in scope, and even create references to it. In this example, "increment_count" and "return_count" share the variable. One function adds to the value and the other simply returns the value. They can both access $count, and since it has gone out of scope, there is no other way to access it. BEGIN { my $count = 1; sub increment_count { $count++ } sub return_count { $count } } To declare a file-private variable, you still use a lexical variable. A file is also a scope, so a lexical variable defined in the file cannot be seen from any other file. See "Persistent Private Variables" in perlsub for more information. The discussion of closures in perlref may help you even though we did not use anonymous subroutines in this answer. See "Persistent Private Variables" in perlsub for details. What's the difference between dynamic and lexical (static) scoping? Between local() and my()? "local($x)" saves away the old value of the global variable $x and assigns a new value for the duration of the subroutine *which is visible in other functions called from that subroutine*. This is done at run-time, so is called dynamic scoping. local() always affects global variables, also called package variables or dynamic variables. "my($x)" creates a new variable that is only visible in the current subroutine. This is done at compile-time, so it is called lexical or static scoping. my() always affects private variables, also called lexical variables or (improperly) static(ly scoped) variables. For instance: sub visible { print "var has value $var\n"; } sub dynamic { local $var = 'local'; # new temporary value for the still-global visible(); # variable called $var } sub lexical { my $var = 'private'; # new private variable, $var visible(); # (invisible outside of sub scope) } $var = 'global'; visible(); # prints global dynamic(); # prints local lexical(); # prints global Notice how at no point does the value "private" get printed. That's because $var only has that value within the block of the lexical() function, and it is hidden from the called subroutine. In summary, local() doesn't make what you think of as private, local variables. It gives a global variable a temporary value. my() is what you're looking for if you want private variables. See "Private Variables via my()" in perlsub and "Temporary Values via local()" in perlsub for excruciating details. How can I access a dynamic variable while a similarly named lexical is in scope? If you know your package, you can just mention it explicitly, as in $Some_Pack::var. Note that the notation $::var is not the dynamic $var in the current package, but rather the one in the "main" package, as though you had written $main::var. use vars '$var'; local $var = "global"; my $var = "lexical"; print "lexical is $var\n"; print "global is $main::var\n"; Alternatively you can use the compiler directive our() to bring a dynamic variable into the current lexical scope. require 5.006; # our() did not exist before 5.6 use vars '$var'; local $var = "global"; my $var = "lexical"; print "lexical is $var\n"; { our $var; print "global is $var\n"; } What's the difference between deep and shallow binding? In deep binding, lexical variables mentioned in anonymous subroutines are the same ones that were in scope when the subroutine was created. In shallow binding, they are whichever variables with the same names happen to be in scope when the subroutine is called. Perl always uses deep binding of lexical variables (i.e., those created with my()). However, dynamic variables (aka global, local, or package variables) are effectively shallowly bound. Consider this just one more reason not to use them. See the answer to "What's a closure?". Why doesn't "my($foo) = <$fh>;" work right? "my()" and "local()" give list context to the right hand side of "=". The <$fh> read operation, like so many of Perl's functions and operators, can tell which context it was called in and behaves appropriately. In general, the scalar() function can help. This function does nothing to the data itself (contrary to popular myth) but rather tells its argument to behave in whatever its scalar fashion is. If that function doesn't have a defined scalar behavior, this of course doesn't help you (such as with sort()). To enforce scalar context in this particular case, however, you need merely omit the parentheses: local($foo) = <$fh>; # WRONG local($foo) = scalar(<$fh>); # ok local $foo = <$fh>; # right You should probably be using lexical variables anyway, although the issue is the same here: my($foo) = <$fh>; # WRONG my $foo = <$fh>; # right How do I redefine a builtin function, operator, or method? Why do you want to do that? :-) If you want to override a predefined function, such as open(), then you'll have to import the new definition from a different module. See "Overriding Built-in Functions" in perlsub. If you want to overload a Perl operator, such as "+" or "**", then you'll want to use the "use overload" pragma, documented in overload. If you're talking about obscuring method calls in parent classes, see "Overriding methods and method resolution" in perlootut. What's the difference between calling a function as &foo and foo()? (contributed by brian d foy) Calling a subroutine as &foo with no trailing parentheses ignores the prototype of "foo" and passes it the current value of the argument list, @_. Here's an example; the "bar" subroutine calls &foo, which prints its arguments list: sub foo { print "Args in foo are: @_\n"; } sub bar { &foo; } bar( "a", "b", "c" ); When you call "bar" with arguments, you see that "foo" got the same @_: Args in foo are: a b c Calling the subroutine with trailing parentheses, with or without arguments, does not use the current @_. Changing the example to put parentheses after the call to "foo" changes the program: sub foo { print "Args in foo are: @_\n"; } sub bar { &foo(); } bar( "a", "b", "c" ); Now the output shows that "foo" doesn't get the @_ from its caller. Args in foo are: However, using "&" in the call still overrides the prototype of "foo" if present: sub foo ($$$) { print "Args infoo are: @_\n"; } sub bar_1 { &foo; } sub bar_2 { &foo(); } sub bar_3 { foo( $_[0], $_[1], $_[2] ); } # sub bar_4 { foo(); } # bar_4 doesn't compile: "Not enough arguments for main::foo at ..." bar_1( "a", "b", "c" ); # Args in foo are: a b c bar_2( "a", "b", "c" ); # Args in foo are: bar_3( "a", "b", "c" ); # Args in foo are: a b c The main use of the @_ pass-through feature is to write subroutines whose main job it is to call other subroutines for you. For further details, see perlsub. How can I catch accesses to undefined variables, functions, or methods? The AUTOLOAD method, discussed in "Autoloading" in perlsub lets you capture calls to undefined functions and methods. When it comes to undefined variables that would trigger a warning under "use warnings", you can promote the warning to an error. use warnings FATAL => qw(uninitialized); Why can't a method included in this same file be found? Some possible reasons: your inheritance is getting confused, you've misspelled the method name, or the object is of the wrong type. Check out perlootut for details about any of the above cases. You may also use "print ref($object)" to find out the class $object was blessed into. Another possible reason for problems is that you've used the indirect object syntax (eg, "find Guru "Samy"") on a class name before Perl has seen that such a package exists. It's wisest to make sure your packages are all defined before you start using them, which will be taken care of if you use the "use" statement instead of "require". If not, make sure to use arrow notation (eg., "Guru->find("Samy")") instead. Object notation is explained in perlobj. Make sure to read about creating modules in perlmod and the perils of indirect objects in "Method Invocation" in perlobj. How can I find out my current or calling package? (contributed by brian d foy) To find the package you are currently in, use the special literal "__PACKAGE__", as documented in perldata. You can only use the special literals as separate tokens, so you can't interpolate them into strings like you can with variables: my $current_package = __PACKAGE__; print "I am in package $current_package\n"; If you want to find the package calling your code, perhaps to give better diagnostics as Carp does, use the "caller" built-in: sub foo { my @args = ...; my( $package, $filename, $line ) = caller; print "I was called from package $package\n"; ); By default, your program starts in package "main", so you will always be in some package. This is different from finding out the package an object is blessed into, which might not be the current package. For that, use "blessed" from Scalar::Util, part of the Standard Library since Perl 5.8: use Scalar::Util qw(blessed); my $object_package = blessed( $object ); Most of the time, you shouldn't care what package an object is blessed into, however, as long as it claims to inherit from that class: my $is_right_class = eval { $object->isa( $package ) }; # true or false And, with Perl 5.10 and later, you don't have to check for an inheritance to see if the object can handle a role. For that, you can use "DOES", which comes from "UNIVERSAL": my $class_does_it = eval { $object->DOES( $role ) }; # true or false You can safely replace "isa" with "DOES" (although the converse is not true). How can I comment out a large block of Perl code? (contributed by brian d foy) The quick-and-dirty way to comment out more than one line of Perl is to surround those lines with Pod directives. You have to put these directives at the beginning of the line and somewhere where Perl expects a new statement (so not in the middle of statements like the "#" comments). You end the comment with "=cut", ending the Pod section: =pod my $object = NotGonnaHappen->new(); ignored_sub(); $wont_be_assigned = 37; =cut The quick-and-dirty method only works well when you don't plan to leave the commented code in the source. If a Pod parser comes along, your multiline comment is going to show up in the Pod translation. A better way hides it from Pod parsers as well. The "=begin" directive can mark a section for a particular purpose. If the Pod parser doesn't want to handle it, it just ignores it. Label the comments with "comment". End the comment using "=end" with the same label. You still need the "=cut" to go back to Perl code from the Pod comment: =begin comment my $object = NotGonnaHappen->new(); ignored_sub(); $wont_be_assigned = 37; =end comment =cut For more information on Pod, check out perlpod and perlpodspec. How do I clear a package? Use this code, provided by Mark-Jason Dominus: sub scrub_package { no strict 'refs'; my $pack = shift; die "Shouldn't delete main package" if $pack eq "" || $pack eq "main"; my $stash = *{$pack . '::'}{HASH}; my $name; foreach $name (keys %$stash) { my $fullname = $pack . '::' . $name; # Get rid of everything with that name. undef $$fullname; undef @$fullname; undef %$fullname; undef &$fullname; undef *$fullname; } } Or, if you're using a recent release of Perl, you can just use the Symbol::delete_package() function instead. How can I use a variable as a variable name? Beginners often think they want to have a variable contain the name of a variable. $fred = 23; $varname = "fred"; ++$$varname; # $fred now 24 This works *sometimes*, but it is a very bad idea for two reasons. The first reason is that this technique *only works on global variables*. That means that if $fred is a lexical variable created with my() in the above example, the code wouldn't work at all: you'd accidentally access the global and skip right over the private lexical altogether. Global variables are bad because they can easily collide accidentally and in general make for non-scalable and confusing code. Symbolic references are forbidden under the "use strict" pragma. They are not true references and consequently are not reference-counted or garbage-collected. The other reason why using a variable to hold the name of another variable is a bad idea is that the question often stems from a lack of understanding of Perl data structures, particularly hashes. By using symbolic references, you are just using the package's symbol-table hash (like %main::) instead of a user-defined hash. The solution is to use your own hash or a real reference instead. $USER_VARS{"fred"} = 23; my $varname = "fred"; $USER_VARS{$varname}++; # not $$varname++ There we're using the %USER_VARS hash instead of symbolic references. Sometimes this comes up in reading strings from the user with variable references and wanting to expand them to the values of your perl program's variables. This is also a bad idea because it conflates the program-addressable namespace and the user-addressable one. Instead of reading a string and expanding it to the actual contents of your program's own variables: $str = 'this has a $fred and $barney in it'; $str =~ s/(\$\w+)/$1/eeg; # need double eval it would be better to keep a hash around like %USER_VARS and have variable references actually refer to entries in that hash: $str =~ s/\$(\w+)/$USER_VARS{$1}/g; # no /e here at all That's faster, cleaner, and safer than the previous approach. Of course, you don't need to use a dollar sign. You could use your own scheme to make it less confusing, like bracketed percent symbols, etc. $str = 'this has a %fred% and %barney% in it'; $str =~ s/%(\w+)%/$USER_VARS{$1}/g; # no /e here at all Another reason that folks sometimes think they want a variable to contain the name of a variable is that they don't know how to build proper data structures using hashes. For example, let's say they wanted two hashes in their program: %fred and %barney, and that they wanted to use another scalar variable to refer to those by name. $name = "fred"; $$name{WIFE} = "wilma"; # set %fred $name = "barney"; $$name{WIFE} = "betty"; # set %barney This is still a symbolic reference, and is still saddled with the problems enumerated above. It would be far better to write: $folks{"fred"}{WIFE} = "wilma"; $folks{"barney"}{WIFE} = "betty"; And just use a multilevel hash to start with. The only times that you absolutely *must* use symbolic references are when you really must refer to the symbol table. This may be because it's something that one can't take a real reference to, such as a format name. Doing so may also be important for method calls, since these always go through the symbol table for resolution. In those cases, you would turn off "strict 'refs'" temporarily so you can play around with the symbol table. For example: @colors = qw(red blue green yellow orange purple violet); for my $name (@colors) { no strict 'refs'; # renege for the block *$name = sub { "<FONT COLOR='$name'>@_</FONT>" }; } All those functions (red(), blue(), green(), etc.) appear to be separate, but the real code in the closure actually was compiled only once. So, sometimes you might want to use symbolic references to manipulate the symbol table directly. This doesn't matter for formats, handles, and subroutines, because they are always global--you can't use my() on them. For scalars, arrays, and hashes, though--and usually for subroutines-- you probably only want to use hard references. Do I need to recompile XS modules when there is a change in the C library? (contributed by Alex Beamish) If the new version of the C library is ABI-compatible (that's Application Binary Interface compatible) with the version you're upgrading from, and if the shared library version didn't change, no re-compilation should be necessary. Found in /usr/share/perl/5.34/pod/perlfaq8.pod How do I print something out in color? In general, you don't, because you don't know whether the recipient has a color-aware display device. If you know that they have an ANSI terminal that understands color, you can use the Term::ANSIColor module from CPAN: use Term::ANSIColor; print color("red"), "Stop!\n", color("reset"); print color("green"), "Go!\n", color("reset"); Or like this: use Term::ANSIColor qw(:constants); print RED, "Stop!\n", RESET; print GREEN, "Go!\n", RESET; How do I clear the screen? (contributed by brian d foy) To clear the screen, you just have to print the special sequence that tells the terminal to clear the screen. Once you have that sequence, output it when you want to clear the screen. You can use the Term::ANSIScreen module to get the special sequence. Import the "cls" function (or the ":screen" tag): use Term::ANSIScreen qw(cls); my $clear_screen = cls(); print $clear_screen; The Term::Cap module can also get the special sequence if you want to deal with the low-level details of terminal control. The "Tputs" method returns the string for the given capability: use Term::Cap; my $terminal = Term::Cap->Tgetent( { OSPEED => 9600 } ); my $clear_screen = $terminal->Tputs('cl'); print $clear_screen; On Windows, you can use the Win32::Console module. After creating an object for the output filehandle you want to affect, call the "Cls" method: Win32::Console; my $OUT = Win32::Console->new(STD_OUTPUT_HANDLE); my $clear_string = $OUT->Cls; print $clear_screen; If you have a command-line program that does the job, you can call it in backticks to capture whatever it outputs so you can use it later: my $clear_string = `clear`; print $clear_string; How do I read and write the serial port? This depends on which operating system your program is running on. In the case of Unix, the serial ports will be accessible through files in "/dev"; on other systems, device names will doubtless differ. Several problem areas common to all device interaction are the following: lockfiles Your system may use lockfiles to control multiple access. Make sure you follow the correct protocol. Unpredictable behavior can result from multiple processes reading from one device. open mode If you expect to use both read and write operations on the device, you'll have to open it for update (see "open" in perlfunc for details). You may wish to open it without running the risk of blocking by using "sysopen()" and "O_RDWR|O_NDELAY|O_NOCTTY" from the Fcntl module (part of the standard perl distribution). See "sysopen" in perlfunc for more on this approach. end of line Some devices will be expecting a "\r" at the end of each line rather than a "\n". In some ports of perl, "\r" and "\n" are different from their usual (Unix) ASCII values of "\015" and "\012". You may have to give the numeric values you want directly, using octal ("\015"), hex ("0x0D"), or as a control-character specification ("\cM"). print DEV "atv1\012"; # wrong, for some devices print DEV "atv1\015"; # right, for some devices Even though with normal text files a "\n" will do the trick, there is still no unified scheme for terminating a line that is portable between Unix, DOS/Win, and Macintosh, except to terminate *ALL* line ends with "\015\012", and strip what you don't need from the output. This applies especially to socket I/O and autoflushing, discussed next. flushing output If you expect characters to get to your device when you "print()" them, you'll want to autoflush that filehandle. You can use "select()" and the $| variable to control autoflushing (see "$|" in perlvar and "select" in perlfunc, or perlfaq5, "How do I flush/unbuffer an output filehandle? Why must I do this?"): my $old_handle = select($dev_fh); $| = 1; select($old_handle); You'll also see code that does this without a temporary variable, as in select((select($deb_handle), $| = 1)[0]); Or if you don't mind pulling in a few thousand lines of code just because you're afraid of a little $| variable: use IO::Handle; $dev_fh->autoflush(1); As mentioned in the previous item, this still doesn't work when using socket I/O between Unix and Macintosh. You'll need to hard code your line terminators, in that case. non-blocking input If you are doing a blocking "read()" or "sysread()", you'll have to arrange for an alarm handler to provide a timeout (see "alarm" in perlfunc). If you have a non-blocking open, you'll likely have a non-blocking read, which means you may have to use a 4-arg "select()" to determine whether I/O is ready on that device (see "select" in perlfunc. While trying to read from his caller-id box, the notorious Jamie Zawinski "<jwz AT netscape.com>", after much gnashing of teeth and fighting with "sysread", "sysopen", POSIX's "tcgetattr" business, and various other functions that go bump in the night, finally came up with this: sub open_modem { use IPC::Open2; my $stty = `/bin/stty -g`; open2( \*MODEM_IN, \*MODEM_OUT, "cu -l$modem_device -s2400 2>&1"); # starting cu hoses /dev/tty's stty settings, even when it has # been opened on a pipe... system("/bin/stty $stty"); $_ = <MODEM_IN>; chomp; if ( !m/^Connected/ ) { print STDERR "$0: cu printed `$_' instead of `Connected'\n"; } } How do I decode encrypted password files? You spend lots and lots of money on dedicated hardware, but this is bound to get you talked about. Seriously, you can't if they are Unix password files--the Unix password system employs one-way encryption. It's more like hashing than encryption. The best you can do is check whether something else hashes to the same string. You can't turn a hash back into the original string. Programs like Crack can forcibly (and intelligently) try to guess passwords, but don't (can't) guarantee quick success. If you're worried about users selecting bad passwords, you should proactively check when they try to change their password (by modifying passwd(1), for example). How do I trap control characters/signals? You don't actually "trap" a control character. Instead, that character generates a signal which is sent to your terminal's currently foregrounded process group, which you then trap in your process. Signals are documented in "Signals" in perlipc and the section on "Signals" in the Camel. You can set the values of the %SIG hash to be the functions you want to handle the signal. After perl catches the signal, it looks in %SIG for a key with the same name as the signal, then calls the subroutine value for that key. # as an anonymous subroutine $SIG{INT} = sub { syswrite(STDERR, "ouch\n", 5 ) }; # or a reference to a function $SIG{INT} = \&ouch; # or the name of the function as a string $SIG{INT} = "ouch"; Perl versions before 5.8 had in its C source code signal handlers which would catch the signal and possibly run a Perl function that you had set in %SIG. This violated the rules of signal handling at that level causing perl to dump core. Since version 5.8.0, perl looks at %SIG after the signal has been caught, rather than while it is being caught. Previous versions of this answer were incorrect. How do I modify the shadow password file on a Unix system? If perl was installed correctly and your shadow library was written properly, the "getpw*()" functions described in perlfunc should in theory provide (read-only) access to entries in the shadow password file. To change the file, make a new shadow password file (the format varies from system to system--see passwd(1) for specifics) and use pwd_mkdb(8) to install it (see pwd_mkdb(8) for more details). How can I sleep() or alarm() for under a second? If you want finer granularity than the 1 second that the "sleep()" function provides, the easiest way is to use the "select()" function as documented in "select" in perlfunc. Try the Time::HiRes and the BSD::Itimer modules (available from CPAN, and starting from Perl 5.8 Time::HiRes is part of the standard distribution). How can I do an atexit() or setjmp()/longjmp()? (Exception handling) You can use the "END" block to simulate "atexit()". Each package's "END" block is called when the program or thread ends. See the perlmod manpage for more details about "END" blocks. For example, you can use this to make sure your filter program managed to finish its output without filling up the disk: END { close(STDOUT) || die "stdout close failed: $!"; } The "END" block isn't called when untrapped signals kill the program, though, so if you use "END" blocks you should also use use sigtrap qw(die normal-signals); Perl's exception-handling mechanism is its "eval()" operator. You can use "eval()" as "setjmp" and "die()" as "longjmp". For details of this, see the section on signals, especially the time-out handler for a blocking "flock()" in "Signals" in perlipc or the section on "Signals" in *Programming Perl*. If exception handling is all you're interested in, use one of the many CPAN modules that handle exceptions, such as Try::Tiny. If you want the "atexit()" syntax (and an "rmexit()" as well), try the "AtExit" module available from CPAN. Why doesn't my sockets program work under System V (Solaris)? What does the error message "Protocol not supported" mean? Some Sys-V based systems, notably Solaris 2.X, redefined some of the standard socket constants. Since these were constant across all architectures, they were often hardwired into perl code. The proper way to deal with this is to "use Socket" to get the correct values. Note that even though SunOS and Solaris are binary compatible, these values are different. Go figure. How can I call my system's unique C functions from Perl? In most cases, you write an external module to do it--see the answer to "Where can I learn about linking C with Perl? [h2xs, xsubpp]". However, if the function is a system call, and your system supports "syscall()", you can use the "syscall" function (documented in perlfunc). Remember to check the modules that came with your distribution, and CPAN as well--someone may already have written a module to do it. On Windows, try Win32::API. On Macs, try Mac::Carbon. If no module has an interface to the C function, you can inline a bit of C in your Perl source with Inline::C. Where do I get the include files to do ioctl() or syscall()? Historically, these would be generated by the h2ph tool, part of the standard perl distribution. This program converts cpp(1) directives in C header files to files containing subroutine definitions, like "SYS_getitimer()", which you can use as arguments to your functions. It doesn't work perfectly, but it usually gets most of the job done. Simple files like errno.h, syscall.h, and socket.h were fine, but the hard ones like ioctl.h nearly always need to be hand-edited. Here's how to install the *.ph files: 1. Become the super-user 2. cd /usr/include 3. h2ph *.h */*.h If your system supports dynamic loading, for reasons of portability and sanity you probably ought to use h2xs (also part of the standard perl distribution). This tool converts C header files to Perl extensions. See perlxstut for how to get started with h2xs. If your system doesn't support dynamic loading, you still probably ought to use h2xs. See perlxstut and ExtUtils::MakeMaker for more information (in brief, just use make perl instead of a plain make to rebuild perl with a new static extension). Why do setuid perl scripts complain about kernel problems? Some operating systems have bugs in the kernel that make setuid scripts inherently insecure. Perl gives you a number of options (described in perlsec) to work around such systems. How can I capture STDERR from an external command? There are three basic ways of running external commands: system $cmd; # using system() my $output = `$cmd`; # using backticks (``) open (my $pipe_fh, "$cmd |"); # using open() With "system()", both STDOUT and STDERR will go the same place as the script's STDOUT and STDERR, unless the "system()" command redirects them. Backticks and "open()" read only the STDOUT of your command. You can also use the "open3()" function from IPC::Open3. Benjamin Goldberg provides some sample code: To capture a program's STDOUT, but discard its STDERR: use IPC::Open3; use File::Spec; my $in = ''; open(NULL, ">", File::Spec->devnull); my $pid = open3($in, \*PH, ">&NULL", "cmd"); while( <PH> ) { } waitpid($pid, 0); To capture a program's STDERR, but discard its STDOUT: use IPC::Open3; use File::Spec; my $in = ''; open(NULL, ">", File::Spec->devnull); my $pid = open3($in, ">&NULL", \*PH, "cmd"); while( <PH> ) { } waitpid($pid, 0); To capture a program's STDERR, and let its STDOUT go to our own STDERR: use IPC::Open3; my $in = ''; my $pid = open3($in, ">&STDERR", \*PH, "cmd"); while( <PH> ) { } waitpid($pid, 0); To read both a command's STDOUT and its STDERR separately, you can redirect them to temp files, let the command run, then read the temp files: use IPC::Open3; use IO::File; my $in = ''; local *CATCHOUT = IO::File->new_tmpfile; local *CATCHERR = IO::File->new_tmpfile; my $pid = open3($in, ">&CATCHOUT", ">&CATCHERR", "cmd"); waitpid($pid, 0); seek $_, 0, 0 for \*CATCHOUT, \*CATCHERR; while( <CATCHOUT> ) {} while( <CATCHERR> ) {} But there's no real need for both to be tempfiles... the following should work just as well, without deadlocking: use IPC::Open3; my $in = ''; use IO::File; local *CATCHERR = IO::File->new_tmpfile; my $pid = open3($in, \*CATCHOUT, ">&CATCHERR", "cmd"); while( <CATCHOUT> ) {} waitpid($pid, 0); seek CATCHERR, 0, 0; while( <CATCHERR> ) {} And it'll be faster, too, since we can begin processing the program's stdout immediately, rather than waiting for the program to finish. With any of these, you can change file descriptors before the call: open(STDOUT, ">logfile"); system("ls"); or you can use Bourne shell file-descriptor redirection: $output = `$cmd 2>some_file`; open (PIPE, "cmd 2>some_file |"); You can also use file-descriptor redirection to make STDERR a duplicate of STDOUT: $output = `$cmd 2>&1`; open (PIPE, "cmd 2>&1 |"); Note that you *cannot* simply open STDERR to be a dup of STDOUT in your Perl program and avoid calling the shell to do the redirection. This doesn't work: open(STDERR, ">&STDOUT"); $alloutput = `cmd args`; # stderr still escapes This fails because the "open()" makes STDERR go to where STDOUT was going at the time of the "open()". The backticks then make STDOUT go to a string, but don't change STDERR (which still goes to the old STDOUT). Note that you *must* use Bourne shell (sh(1)) redirection syntax in backticks, not csh(1)! Details on why Perl's "system()" and backtick and pipe opens all use the Bourne shell are in the versus/csh.whynot article in the "Far More Than You Ever Wanted To Know" collection in <http://www.cpan.org/misc/olddoc/FMTEYEWTK.tgz> . To capture a command's STDERR and STDOUT together: $output = `cmd 2>&1`; # either with backticks $pid = open(PH, "cmd 2>&1 |"); # or with an open pipe while (<PH>) { } # plus a read To capture a command's STDOUT but discard its STDERR: $output = `cmd 2>/dev/null`; # either with backticks $pid = open(PH, "cmd 2>/dev/null |"); # or with an open pipe while (<PH>) { } # plus a read To capture a command's STDERR but discard its STDOUT: $output = `cmd 2>&1 1>/dev/null`; # either with backticks $pid = open(PH, "cmd 2>&1 1>/dev/null |"); # or with an open pipe while (<PH>) { } # plus a read To exchange a command's STDOUT and STDERR in order to capture the STDERR but leave its STDOUT to come out our old STDERR: $output = `cmd 3>&1 1>&2 2>&3 3>&-`; # either with backticks $pid = open(PH, "cmd 3>&1 1>&2 2>&3 3>&-|");# or with an open pipe while (<PH>) { } # plus a read To read both a command's STDOUT and its STDERR separately, it's easiest to redirect them separately to files, and then read from those files when the program is done: system("program args 1>program.stdout 2>program.stderr"); Ordering is important in all these examples. That's because the shell processes file descriptor redirections in strictly left to right order. system("prog args 1>tmpfile 2>&1"); system("prog args 2>&1 1>tmpfile"); The first command sends both standard out and standard error to the temporary file. The second command sends only the old standard output there, and the old standard error shows up on the old standard out. Why doesn't open() return an error when a pipe open fails? If the second argument to a piped "open()" contains shell metacharacters, perl "fork()"s, then "exec()"s a shell to decode the metacharacters and eventually run the desired program. If the program couldn't be run, it's the shell that gets the message, not Perl. All your Perl program can find out is whether the shell itself could be successfully started. You can still capture the shell's STDERR and check it for error messages. See "How can I capture STDERR from an external command?" elsewhere in this document, or use the IPC::Open3 module. If there are no shell metacharacters in the argument of "open()", Perl runs the command directly, without using the shell, and can correctly report whether the command started. How can I call backticks without shell processing? This is a bit tricky. You can't simply write the command like this: @ok = `grep @opts '$search_string' @filenames`; As of Perl 5.8.0, you can use "open()" with multiple arguments. Just like the list forms of "system()" and "exec()", no shell escapes happen. open( GREP, "-|", 'grep', @opts, $search_string, @filenames ); chomp(@ok = <GREP>); close GREP; You can also: my @ok = (); if (open(GREP, "-|")) { while (<GREP>) { chomp; push(@ok, $_); } close GREP; } else { exec 'grep', @opts, $search_string, @filenames; } Just as with "system()", no shell escapes happen when you "exec()" a list. Further examples of this can be found in "Safe Pipe Opens" in perlipc. Note that if you're using Windows, no solution to this vexing issue is even possible. Even though Perl emulates "fork()", you'll still be stuck, because Windows does not have an argc/argv-style API. How can I convert my shell script to perl? Learn Perl and rewrite it. Seriously, there's no simple converter. Things that are awkward to do in the shell are easy to do in Perl, and this very awkwardness is what would make a shell->perl converter nigh-on impossible to write. By rewriting it, you'll think about what you're really trying to do, and hopefully will escape the shell's pipeline datastream paradigm, which while convenient for some matters, causes many inefficiencies. Can I use perl to run a telnet or ftp session? Try the Net::FTP, TCP::Client, and Net::Telnet modules (available from CPAN). <http://www.cpan.org/scripts/netstuff/telnet.emul.shar> will also help for emulating the telnet protocol, but Net::Telnet is quite probably easier to use. If all you want to do is pretend to be telnet but don't need the initial telnet handshaking, then the standard dual-process approach will suffice: use IO::Socket; # new in 5.004 my $handle = IO::Socket::INET->new('www.perl.com:80') or die "can't connect to port 80 on www.perl.com $!"; $handle->autoflush(1); if (fork()) { # XXX: undef means failure select($handle); print while <STDIN>; # everything from stdin to socket } else { print while <$handle>; # everything from socket to stdout } close $handle; exit; How can I write expect in Perl? Once upon a time, there was a library called chat2.pl (part of the standard perl distribution), which never really got finished. If you find it somewhere, *don't use it*. These days, your best bet is to look at the Expect module available from CPAN, which also requires two other modules from CPAN, IO::Pty and IO::Stty. Is there a way to hide perl's command line from programs such as "ps"? First of all note that if you're doing this for security reasons (to avoid people seeing passwords, for example) then you should rewrite your program so that critical information is never given as an argument. Hiding the arguments won't make your program completely secure. To actually alter the visible command line, you can assign to the variable $0 as documented in perlvar. This won't work on all operating systems, though. Daemon programs like sendmail place their state there, as in: $0 = "orcus [accepting connections]"; I {changed directory, modified my environment} in a perl script. How come the change disappeared when I exited the script? How do I get my changes to be visible? Unix In the strictest sense, it can't be done--the script executes as a different process from the shell it was started from. Changes to a process are not reflected in its parent--only in any children created after the change. There is shell magic that may allow you to fake it by "eval()"ing the script's output in your shell; check out the comp.unix.questions FAQ for details. How do I close a process's filehandle without waiting for it to complete? Assuming your system supports such things, just send an appropriate signal to the process (see "kill" in perlfunc). It's common to first send a TERM signal, wait a little bit, and then send a KILL signal to finish it off. How do I find out if I'm running interactively or not? (contributed by brian d foy) This is a difficult question to answer, and the best answer is only a guess. What do you really want to know? If you merely want to know if one of your filehandles is connected to a terminal, you can try the "-t" file test: if( -t STDOUT ) { print "I'm connected to a terminal!\n"; } However, you might be out of luck if you expect that means there is a real person on the other side. With the Expect module, another program can pretend to be a person. The program might even come close to passing the Turing test. The IO::Interactive module does the best it can to give you an answer. Its "is_interactive" function returns an output filehandle; that filehandle points to standard output if the module thinks the session is interactive. Otherwise, the filehandle is a null handle that simply discards the output: use IO::Interactive; print { is_interactive } "I might go to standard output!\n"; This still doesn't guarantee that a real person is answering your prompts or reading your output. If you want to know how to handle automated testing for your distribution, you can check the environment. The CPAN Testers, for instance, set the value of "AUTOMATED_TESTING": unless( $ENV{AUTOMATED_TESTING} ) { print "Hello interactive tester!\n"; } How do I timeout a slow event? Use the "alarm()" function, probably in conjunction with a signal handler, as documented in "Signals" in perlipc and the section on "Signals" in the Camel. You may instead use the more flexible Sys::AlarmCall module available from CPAN. The "alarm()" function is not implemented on all versions of Windows. Check the documentation for your specific version of Perl. How do I set CPU limits? (contributed by Xho) Use the BSD::Resource module from CPAN. As an example: use BSD::Resource; setrlimit(RLIMIT_CPU,10,20) or die $!; This sets the soft and hard limits to 10 and 20 seconds, respectively. After 10 seconds of time spent running on the CPU (not "wall" time), the process will be sent a signal (XCPU on some systems) which, if not trapped, will cause the process to terminate. If that signal is trapped, then after 10 more seconds (20 seconds in total) the process will be killed with a non-trappable signal. See the BSD::Resource and your systems documentation for the gory details. How do I use an SQL database? The DBI module provides an abstract interface to most database servers and types, including Oracle, DB2, Sybase, mysql, Postgresql, ODBC, and flat files. The DBI module accesses each database type through a database driver, or DBD. You can see a complete list of available drivers on CPAN: <http://www.cpan.org/modules/by-module/DBD/> . You can read more about DBI on <http://dbi.perl.org/> . Other modules provide more specific access: Win32::ODBC, Alzabo, "iodbc", and others found on CPAN Search: <https://metacpan.org/> . How do I make a system() exit on control-C? You can't. You need to imitate the "system()" call (see perlipc for sample code) and then have a signal handler for the INT signal that passes the signal on to the subprocess. Or you can check for it: $rc = system($cmd); if ($rc & 127) { die "signal death" } How do I open a file without blocking? If you're lucky enough to be using a system that supports non-blocking reads (most Unixish systems do), you need only to use the "O_NDELAY" or "O_NONBLOCK" flag from the "Fcntl" module in conjunction with "sysopen()": use Fcntl; sysopen(my $fh, "/foo/somefile", O_WRONLY|O_NDELAY|O_CREAT, 0644) or die "can't open /foo/somefile: $!": How do I tell the difference between errors from the shell and perl? (answer contributed by brian d foy) When you run a Perl script, something else is running the script for you, and that something else may output error messages. The script might emit its own warnings and error messages. Most of the time you cannot tell who said what. You probably cannot fix the thing that runs perl, but you can change how perl outputs its warnings by defining a custom warning and die functions. Consider this script, which has an error you may not notice immediately. #!/usr/locl/bin/perl print "Hello World\n"; I get an error when I run this from my shell (which happens to be bash). That may look like perl forgot it has a "print()" function, but my shebang line is not the path to perl, so the shell runs the script, and I get the error. $ ./test ./test: line 3: print: command not found A quick and dirty fix involves a little bit of code, but this may be all you need to figure out the problem. #!/usr/bin/perl -w BEGIN { $SIG{__WARN__} = sub{ print STDERR "Perl: ", @_; }; $SIG{__DIE__} = sub{ print STDERR "Perl: ", @_; exit 1}; } $a = 1 + undef; $x / 0; __END__ The perl message comes out with "Perl" in front. The "BEGIN" block works at compile time so all of the compilation errors and warnings get the "Perl:" prefix too. Perl: Useless use of division (/) in void context at ./test line 9. Perl: Name "main::a" used only once: possible typo at ./test line 8. Perl: Name "main::x" used only once: possible typo at ./test line 9. Perl: Use of uninitialized value in addition (+) at ./test line 8. Perl: Use of uninitialized value in division (/) at ./test line 9. Perl: Illegal division by zero at ./test line 9. Perl: Illegal division by zero at -e line 3. If I don't see that "Perl:", it's not from perl. You could also just know all the perl errors, and although there are some people who may know all of them, you probably don't. However, they all should be in the perldiag manpage. If you don't find the error in there, it probably isn't a perl error. Looking up every message is not the easiest way, so let perl to do it for you. Use the diagnostics pragma with turns perl's normal messages into longer discussions on the topic. use diagnostics; If you don't get a paragraph or two of expanded discussion, it might not be perl's message. How do I install a module from CPAN? (contributed by brian d foy) The easiest way is to have a module also named CPAN do it for you by using the "cpan" command that comes with Perl. You can give it a list of modules to install: $ cpan IO::Interactive Getopt::Whatever If you prefer "CPANPLUS", it's just as easy: $ cpanp i IO::Interactive Getopt::Whatever If you want to install a distribution from the current directory, you can tell "CPAN.pm" to install "." (the full stop): $ cpan . See the documentation for either of those commands to see what else you can do. If you want to try to install a distribution by yourself, resolving all dependencies on your own, you follow one of two possible build paths. For distributions that use *Makefile.PL*: $ perl Makefile.PL $ make test install For distributions that use *Build.PL*: $ perl Build.PL $ ./Build test $ ./Build install Some distributions may need to link to libraries or other third-party code and their build and installation sequences may be more complicated. Check any *README* or *INSTALL* files that you may find. How do I keep my own module/library directory? When you build modules, tell Perl where to install the modules. If you want to install modules for your own use, the easiest way might be local::lib, which you can download from CPAN. It sets various installation settings for you, and uses those same settings within your programs. If you want more flexibility, you need to configure your CPAN client for your particular situation. For "Makefile.PL"-based distributions, use the INSTALL_BASE option when generating Makefiles: perl Makefile.PL INSTALL_BASE=/mydir/perl You can set this in your "CPAN.pm" configuration so modules automatically install in your private library directory when you use the CPAN.pm shell: % cpan cpan> o conf makepl_arg INSTALL_BASE=/mydir/perl cpan> o conf commit For "Build.PL"-based distributions, use the --install_base option: perl Build.PL --install_base /mydir/perl You can configure "CPAN.pm" to automatically use this option too: % cpan cpan> o conf mbuild_arg "--install_base /mydir/perl" cpan> o conf commit INSTALL_BASE tells these tools to put your modules into /mydir/perl/lib/perl5. See "How do I add a directory to my include path (@INC) at runtime?" for details on how to run your newly installed modules. There is one caveat with INSTALL_BASE, though, since it acts differently from the PREFIX and LIB settings that older versions of ExtUtils::MakeMaker advocated. INSTALL_BASE does not support installing modules for multiple versions of Perl or different architectures under the same directory. You should consider whether you really want that and, if you do, use the older PREFIX and LIB settings. See the ExtUtils::Makemaker documentation for more details. How do I add the directory my program lives in to the module/library search path? (contributed by brian d foy) If you know the directory already, you can add it to @INC as you would for any other directory. You might "use lib" if you know the directory at compile time: use lib $directory; The trick in this task is to find the directory. Before your script does anything else (such as a "chdir"), you can get the current working directory with the "Cwd" module, which comes with Perl: BEGIN { use Cwd; our $directory = cwd; } use lib $directory; You can do a similar thing with the value of $0, which holds the script name. That might hold a relative path, but "rel2abs" can turn it into an absolute path. Once you have the BEGIN { use File::Spec::Functions qw(rel2abs); use File::Basename qw(dirname); my $path = rel2abs( $0 ); our $directory = dirname( $path ); } use lib $directory; The FindBin module, which comes with Perl, might work. It finds the directory of the currently running script and puts it in $Bin, which you can then use to construct the right library path: use FindBin qw($Bin); You can also use local::lib to do much of the same thing. Install modules using local::lib's settings then use the module in your program: use local::lib; # sets up a local lib at ~/perl5 See the local::lib documentation for more details. How do I add a directory to my include path (@INC) at runtime? Here are the suggested ways of modifying your include path, including environment variables, run-time switches, and in-code statements: the "PERLLIB" environment variable $ export PERLLIB=/path/to/my/dir $ perl program.pl the "PERL5LIB" environment variable $ export PERL5LIB=/path/to/my/dir $ perl program.pl the "perl -Idir" command line flag $ perl -I/path/to/my/dir program.pl the "lib" pragma: use lib "$ENV{HOME}/myown_perllib"; the local::lib module: use local::lib; use local::lib "~/myown_perllib"; Where are modules installed? Modules are installed on a case-by-case basis (as provided by the methods described in the previous section), and in the operating system. All of these paths are stored in @INC, which you can display with the one-liner perl -e 'print join("\n",@INC,"")' The same information is displayed at the end of the output from the command perl -V To find out where a module's source code is located, use perldoc -l Encode to display the path to the module. In some cases (for example, the "AutoLoader" module), this command will show the path to a separate "pod" file; the module itself should be in the same directory, with a 'pm' file extension. Found in /usr/share/perl/5.34/pod/perlfaq9.pod Should I use a web framework? Yes. If you are building a web site with any level of interactivity (forms / users / databases), you will want to use a framework to make handling requests and responses easier. If there is no interactivity then you may still want to look at using something like Template Toolkit <https://metacpan.org/module/Template> or Plack::Middleware::TemplateToolkit so maintenance of your HTML files (and other assets) is easier. Which web framework should I use? There is no simple answer to this question. Perl frameworks can run everything from basic file servers and small scale intranets to massive multinational multilingual websites that are the core to international businesses. Below is a list of a few frameworks with comments which might help you in making a decision, depending on your specific requirements. Start by reading the docs, then ask questions on the relevant mailing list or IRC channel. Catalyst Strongly object-oriented and fully-featured with a long development history and a large community and addon ecosystem. It is excellent for large and complex applications, where you have full control over the server. Dancer2 Free of legacy weight, providing a lightweight and easy to learn API. Has a growing addon ecosystem. It is best used for smaller projects and very easy to learn for beginners. Mojolicious Self-contained and powerful for both small and larger projects, with a focus on HTML5 and real-time web technologies such as WebSockets. Web::Simple Strongly object-oriented and minimal, built for speed and intended as a toolkit for building micro web apps, custom frameworks or for tieing together existing Plack-compatible web applications with one central dispatcher. All of these interact with or use Plack which is worth understanding the basics of when building a website in Perl (there is a lot of useful Plack::Middleware <https://metacpan.org/search?q=plack%3A%3Amiddleware>). What is Plack and PSGI? PSGI is the Perl Web Server Gateway Interface Specification, it is a standard that many Perl web frameworks use, you should not need to understand it to build a web site, the part you might want to use is Plack. Plack is a set of tools for using the PSGI stack. It contains middleware <https://metacpan.org/search?q=plack%3A%3Amiddleware> components, a reference server and utilities for Web application frameworks. Plack is like Ruby's Rack or Python's Paste for WSGI. You could build a web site using Plack and your own code, but for anything other than a very basic web site, using a web framework (that uses <https://plackperl.org>) is a better option. How do I remove HTML from a string? Use HTML::Strip, or HTML::FormatText which not only removes HTML but also attempts to do a little simple formatting of the resulting plain text. How do I extract URLs? HTML::SimpleLinkExtor will extract URLs from HTML, it handles anchors, images, objects, frames, and many other tags that can contain a URL. If you need anything more complex, you can create your own subclass of HTML::LinkExtor or HTML::Parser. You might even use HTML::SimpleLinkExtor as an example for something specifically suited to your needs. You can use URI::Find or URL::Search to extract URLs from an arbitrary text document. How do I fetch an HTML file? (contributed by brian d foy) The core HTTP::Tiny module can fetch web resources and give their content back to you as a string: use HTTP::Tiny; my $ua = HTTP::Tiny->new; my $html = $ua->get( "http://www.example.com/index.html" )->{content}; It can also store the resource directly in a file: $ua->mirror( "http://www.example.com/index.html", "foo.html" ); If you need to do something more complicated, the HTTP::Tiny object can be customized by setting attributes, or you can use LWP::UserAgent from the libwww-perl distribution or Mojo::UserAgent from the Mojolicious distribution to make common tasks easier. If you want to simulate an interactive web browser, you can use the WWW::Mechanize module. How do I automate an HTML form submission? If you are doing something complex, such as moving through many pages and forms or a web site, you can use WWW::Mechanize. See its documentation for all the details. If you're submitting values using the GET method, create a URL and encode the form using the "www_form_urlencode" method from HTTP::Tiny: use HTTP::Tiny; my $ua = HTTP::Tiny->new; my $query = $ua->www_form_urlencode([ q => 'DB_File', lucky => 1 ]); my $url = "https://metacpan.org/search?$query"; my $content = $ua->get($url)->{content}; If you're using the POST method, the "post_form" method will encode the content appropriately. use HTTP::Tiny; my $ua = HTTP::Tiny->new; my $url = 'https://metacpan.org/search'; my $form = [ q => 'DB_File', lucky => 1 ]; my $content = $ua->post_form($url, $form)->{content}; How do I make sure users can't enter values into a form that causes my CGI script to do bad things? (contributed by brian d foy) You can't prevent people from sending your script bad data. Even if you add some client-side checks, people may disable them or bypass them completely. For instance, someone might use a module such as LWP to submit to your web site. If you want to prevent data that try to use SQL injection or other sorts of attacks (and you should want to), you have to not trust any data that enter your program. The perlsec documentation has general advice about data security. If you are using the DBI module, use placeholder to fill in data. If you are running external programs with "system" or "exec", use the list forms. There are many other precautions that you should take, too many to list here, and most of them fall under the category of not using any data that you don't intend to use. Trust no one. How do I parse a mail header? Use the Email::MIME module. It's well-tested and supports all the craziness that you'll see in the real world (comment-folding whitespace, encodings, comments, etc.). use Email::MIME; my $message = Email::MIME->new($rfc2822); my $subject = $message->header('Subject'); my $from = $message->header('From'); If you've already got some other kind of email object, consider passing it to Email::Abstract and then using its cast method to get an Email::MIME object: my $abstract = Email::Abstract->new($mail_message_object); my $email_mime_object = $abstract->cast('Email::MIME'); How do I check a valid mail address? (partly contributed by Aaron Sherman) This isn't as simple a question as it sounds. There are two parts: a) How do I verify that an email address is correctly formatted? b) How do I verify that an email address targets a valid recipient? Without sending mail to the address and seeing whether there's a human on the other end to answer you, you cannot fully answer part *b*, but the Email::Valid module will do both part *a* and part *b* as far as you can in real-time. Our best advice for verifying a person's mail address is to have them enter their address twice, just as you normally do to change a password. This usually weeds out typos. If both versions match, send mail to that address with a personal message. If you get the message back and they've followed your directions, you can be reasonably assured that it's real. A related strategy that's less open to forgery is to give them a PIN (personal ID number). Record the address and PIN (best that it be a random one) for later processing. In the mail you send, include a link to your site with the PIN included. If the mail bounces, you know it's not valid. If they don't click on the link, either they forged the address or (assuming they got the message) following through wasn't important so you don't need to worry about it. How do I find the user's mail address? Ask them for it. There are so many email providers available that it's unlikely the local system has any idea how to determine a user's email address. The exception is for organization-specific email (e.g. foo AT yourcompany.com) where policy can be codified in your program. In that case, you could look at $ENV{USER}, $ENV{LOGNAME}, and getpwuid($<) in scalar context, like so: my $user_name = getpwuid($<) But you still cannot make assumptions about whether this is correct, unless your policy says it is. You really are best off asking the user. How do I send email? Use the Email::Stuffer module, like so: # first, create your message my $message = Email::Stuffer->from('you AT example.com') ->to('friend AT example.com') ->subject('Happy birthday!') ->text_body("Happy birthday to you!\n"); $message->send_or_die; By default, Email::Sender::Simple (the "send" and "send_or_die" methods use this under the hood) will try "sendmail" first, if it exists in your $PATH. This generally isn't the case. If there's a remote mail server you use to send mail, consider investigating one of the Transport classes. At time of writing, the available transports include: Email::Sender::Transport::Sendmail This is the default. If you can use the mail(1) or mailx(1) program to send mail from the machine where your code runs, you should be able to use this. Email::Sender::Transport::SMTP This transport contacts a remote SMTP server over TCP. It optionally uses TLS or SSL and can authenticate to the server via SASL. Telling Email::Stuffer to use your transport is straightforward. $message->transport($email_sender_transport_object)->send_or_die; How do I use MIME to make an attachment to a mail message? Email::MIME directly supports multipart messages. Email::MIME objects themselves are parts and can be attached to other Email::MIME objects. Consult the Email::MIME documentation for more information, including all of the supported methods and examples of their use. Email::Stuffer uses Email::MIME under the hood to construct messages, and wraps the most common attachment tasks with the simple "attach" and "attach_file" methods. Email::Stuffer->to('friend AT example.com') ->subject('The file') ->attach_file('stuff.csv') ->send_or_die; How do I read email? Use the Email::Folder module, like so: use Email::Folder; my $folder = Email::Folder->new('/path/to/email/folder'); while(my $message = $folder->next_message) { # next_message returns Email::Simple objects, but we want # Email::MIME objects as they're more robust my $mime = Email::MIME->new($message->as_string); } There are different classes in the Email::Folder namespace for supporting various mailbox types. Note that these modules are generally rather limited and only support reading rather than writing. How do I fetch/put an (S)FTP file? Net::FTP, and Net::SFTP allow you to interact with FTP and SFTP (Secure FTP) servers. How can I do RPC in Perl? Use one of the RPC modules( <https://metacpan.org/search?q=RPC> ).
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