man > perlform(1)

PERLFORM(1)                       Perl Programmers Reference Guide                       PERLFORM(1)

NAME
       perlform - Perl formats

DESCRIPTION
       Perl has a mechanism to help you generate simple reports and charts.  To facilitate this,
       Perl helps you code up your output page close to how it will look when it's printed.  It can
       keep track of things like how many lines are on a page, what page you're on, when to print
       page headers, etc.  Keywords are borrowed from FORTRAN: format() to declare and write() to
       execute; see their entries in perlfunc.  Fortunately, the layout is much more legible, more
       like BASIC's PRINT USING statement.  Think of it as a poor man's nroff(1).

       Formats, like packages and subroutines, are declared rather than executed, so they may occur
       at any point in your program.  (Usually it's best to keep them all together though.) They
       have their own namespace apart from all the other "types" in Perl.  This means that if you
       have a function named "Foo", it is not the same thing as having a format named "Foo".
       However, the default name for the format associated with a given filehandle is the same as
       the name of the filehandle.  Thus, the default format for STDOUT is named "STDOUT", and the
       default format for filehandle TEMP is named "TEMP".  They just look the same.  They aren't.

       Output record formats are declared as follows:

           format NAME =
           FORMLIST
           .

       If the name is omitted, format "STDOUT" is defined. A single "." in column 1 is used to
       terminate a format.  FORMLIST consists of a sequence of lines, each of which may be one of
       three types:

       1.  A comment, indicated by putting a '#' in the first column.

       2.  A "picture" line giving the format for one output line.

       3.  An argument line supplying values to plug into the previous picture line.

       Picture  lines  contain output field definitions, intermingled with literal text. These lines
       do not undergo any kind of variable interpolation.  Field definitions are made up from a  set
       of  characters, for starting and extending a field to its desired width. This is the complete
       set of characters for field definitions:

          @    start of regular field
          ^    start of special field
          <    pad character for left justification
          |    pad character for centering
          >    pad character for right justification
          #    pad character for a right-justified numeric field
          0    instead of first #: pad number with leading zeroes
          .    decimal point within a numeric field
          ...  terminate a text field, show "..." as truncation evidence
          @*   variable width field for a multi-line value
          ^*   variable width field for next line of a multi-line value
          ~    suppress line with all fields empty
          ~~   repeat line until all fields are exhausted

       Each field in a picture line starts with either "@" (at)  or  "^"  (caret),  indicating  what
       we'll  call,  respectively,  a  "regular"  or  "special" field.  The choice of pad characters
       determines whether a field is textual or numeric. The tilde  operators  are  not  part  of  a
       field.  Let's look at the various possibilities in detail.

   Text Fields
       The  length  of the field is supplied by padding out the field with multiple "<", ">", or "|"
       characters to specify a non-numeric  field  with,  respectively,  left  justification,  right
       justification,  or  centering.   For  a regular field, the value (up to the first newline) is
       taken and printed according to the selected justification, truncating excess characters.   If
       you  terminate a text field with "...", three dots will be shown if the value is truncated. A
       special text field may be used to do rudimentary multi-line text block  filling;  see  "Using
       Fill Mode" for details.

          Example:
             format STDOUT =
             @<<<<<<   @||||||   @>>>>>>
             "left",   "middle", "right"
             .
          Output:
             left      middle    right

   Numeric Fields
       Using  "#"  as  a  padding  character specifies a numeric field, with right justification. An
       optional "." defines the position of the decimal point. With a  "0"  (zero)  instead  of  the
       first  "#",  the formatted number will be padded with leading zeroes if necessary.  A special
       numeric field is blanked out if the value is undefined.  If the resulting value would  exceed
       the width specified the field is filled with "#" as overflow evidence.

          Example:
             format STDOUT =
             @###   @.###   @##.###  @###   @###   ^####
              42,   3.1415,  undef,    0, 10000,   undef
             .
          Output:
               42   3.142     0.000     0   ####

   The Field @* for Variable-Width Multi-Line Text
       The  field "@*" can be used for printing multi-line, nontruncated values; it should (but need
       not) appear by itself on a line. A final line feed is chomped off, but all  other  characters
       are emitted verbatim.

   The Field ^* for Variable-Width One-line-at-a-time Text
       Like "@*", this is a variable-width field. The value supplied must be a scalar variable. Perl
       puts the first line (up to the first "\n") of the text into the field, and then chops off the
       front of the string so that the next time the variable is referenced, more of the text can be
       printed.  The variable will not be restored.

          Example:
             $text = "line 1\nline 2\nline 3";
             format STDOUT =
             Text: ^*
                   $text
             ~~    ^*
                   $text
             .
          Output:
             Text: line 1
                   line 2
                   line 3

   Specifying Values
       The  values  are  specified  on  the  following  format line in the same order as the picture
       fields.  The expressions providing the values must be separated  by  commas.   They  are  all
       evaluated  in  a list context before the line is processed, so a single list expression could
       produce multiple list elements.  The expressions may be spread out to more than one  line  if
       enclosed  in braces.  If so, the opening brace must be the first token on the first line.  If
       an expression evaluates to a number with a decimal part, and  if  the  corresponding  picture
       specifies  that  the  decimal  part  should appear in the output (that is, any picture except
       multiple "#" characters without an embedded "."), the character used for the decimal point is
       determined by the current LC_NUMERIC locale if "use locale" is in effect.  This  means  that,
       if,  for  example,  the  run-time environment happens to specify a German locale, "," will be
       used instead of the default ".".  See perllocale and "WARNINGS" for more information.

   Using Fill Mode
       On text fields the caret enables a kind of fill mode.  Instead of  an  arbitrary  expression,
       the value supplied must be a scalar variable that contains a text string.  Perl puts the next
       portion  of  the  text into the field, and then chops off the front of the string so that the
       next time the variable is referenced, more of the text can be printed.  (Yes, this means that
       the variable itself is altered during execution of the write() call, and  is  not  restored.)
       The  next  portion  of text is determined by a crude line-breaking algorithm. You may use the
       carriage return character ("\r") to force a line break. You can change which  characters  are
       legal to break on by changing the variable $: (that's $FORMAT_LINE_BREAK_CHARACTERS if you're
       using the English module) to a list of the desired characters.

       Normally  you  would  use  a  sequence of fields in a vertical stack associated with the same
       scalar variable to print out a block of text. You might wish to end the final field with  the
       text  "...",  which  will  appear  in  the  output  if the text was too long to appear in its
       entirety.

   Suppressing Lines Where All Fields Are Void
       Using caret fields can produce lines where all fields are blank. You can suppress such  lines
       by  putting  a "~" (tilde) character anywhere in the line.  The tilde will be translated to a
       space upon output.

   Repeating Format Lines
       If you put two contiguous tilde characters "~~" anywhere  into  a  line,  the  line  will  be
       repeated  until all the fields on the line are exhausted, i.e. undefined. For special (caret)
       text fields this will occur sooner or later, but if you use a text field of the  at  variety,
       the   expression you supply had better not give the same value every time forever! (shift(@f)
       is a simple example that would work.)  Don't use a regular (at) numeric field in such  lines,
       because it will never go blank.

   Top of Form Processing
       Top-of-form  processing  is  by default handled by a format with the same name as the current
       filehandle with "_TOP" concatenated to it.  It's triggered at the  top  of  each  page.   See
       "write" in perlfunc.

       Examples:

        # a report on the /etc/passwd file
        format STDOUT_TOP =
                                Passwd File
        Name                Login    Office   Uid   Gid Home
        ------------------------------------------------------------------
        .
        format STDOUT =
        @<<<<<<<<<<<<<<<<<< @||||||| @<<<<<<@>>>> @>>>> @<<<<<<<<<<<<<<<<<
        $name,              $login,  $office,$uid,$gid, $home
        .


        # a report from a bug report form
        format STDOUT_TOP =
                                Bug Reports
        @<<<<<<<<<<<<<<<<<<<<<<<     @|||         @>>>>>>>>>>>>>>>>>>>>>>>
        $system,                      $%,         $date
        ------------------------------------------------------------------
        .
        format STDOUT =
        Subject: @<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<
                 $subject
        Index: @<<<<<<<<<<<<<<<<<<<<<<<<<<<< ^<<<<<<<<<<<<<<<<<<<<<<<<<<<<
               $index,                       $description
        Priority: @<<<<<<<<<< Date: @<<<<<<< ^<<<<<<<<<<<<<<<<<<<<<<<<<<<<
                  $priority,        $date,   $description
        From: @<<<<<<<<<<<<<<<<<<<<<<<<<<<<< ^<<<<<<<<<<<<<<<<<<<<<<<<<<<<
              $from,                         $description
        Assigned to: @<<<<<<<<<<<<<<<<<<<<<< ^<<<<<<<<<<<<<<<<<<<<<<<<<<<<
                     $programmer,            $description
        ~                                    ^<<<<<<<<<<<<<<<<<<<<<<<<<<<<
                                             $description
        ~                                    ^<<<<<<<<<<<<<<<<<<<<<<<<<<<<
                                             $description
        ~                                    ^<<<<<<<<<<<<<<<<<<<<<<<<<<<<
                                             $description
        ~                                    ^<<<<<<<<<<<<<<<<<<<<<<<<<<<<
                                             $description
        ~                                    ^<<<<<<<<<<<<<<<<<<<<<<<...
                                             $description
        .

       It is possible to intermix print()s with write()s on the same output channel, but you'll have
       to handle "$-" ($FORMAT_LINES_LEFT) yourself.

   Format Variables
       The  current  format name is stored in the variable $~ ($FORMAT_NAME), and the current top of
       form format name is in $^ ($FORMAT_TOP_NAME).  The current output page number is stored in $%
       ($FORMAT_PAGE_NUMBER), and the number of lines on the page is in $= ($FORMAT_LINES_PER_PAGE).
       Whether to autoflush output on this handle is stored in $| ($OUTPUT_AUTOFLUSH).   The  string
       output before each top of page (except the first) is stored in $^L ($FORMAT_FORMFEED).  These
       variables  are set on a per-filehandle basis, so you'll need to select() into a different one
       to affect them:

           select((select(OUTF),
                   $~ = "My_Other_Format",
                   $^ = "My_Top_Format"
                  )[0]);

       Pretty ugly, eh?  It's a common idiom though, so don't be too surprised when you see it.  You
       can at least use a temporary variable to hold the previous filehandle: (this is a much better
       approach in general, because not only does legibility improve, you now have  an  intermediary
       stage in the expression to single-step the debugger through):

           $ofh = select(OUTF);
           $~ = "My_Other_Format";
           $^ = "My_Top_Format";
           select($ofh);

       If you use the English module, you can even read the variable names:

           use English;
           $ofh = select(OUTF);
           $FORMAT_NAME     = "My_Other_Format";
           $FORMAT_TOP_NAME = "My_Top_Format";
           select($ofh);

       But  you  still have those funny select()s.  So just use the FileHandle module.  Now, you can
       access these special variables using lowercase method names instead:

           use FileHandle;
           format_name     OUTF "My_Other_Format";
           format_top_name OUTF "My_Top_Format";

       Much better!

NOTES
       Because the values line may contain arbitrary expressions (for at fields, not caret  fields),
       you  can  farm out more sophisticated processing to other functions, like sprintf() or one of
       your own.  For example:

           format Ident =
               @<<<<<<<<<<<<<<<
               &commify($n)
           .

       To get a real at or caret into the field, do this:

           format Ident =
           I have an @ here.
                   "@"
           .

       To center a whole line of text, do something like this:

           format Ident =
           @|||||||||||||||||||||||||||||||||||||||||||||||
                   "Some text line"
           .

       There is no builtin way to say "float this to the right hand side of the page,  however  wide
       it  is."   You  have  to  specify  where it goes.  The truly desperate can generate their own
       format on the fly, based on the current number of columns, and then eval() it:

           $format  = "format STDOUT = \n"
                    . '^' . '<' x $cols . "\n"
                    . '$entry' . "\n"
                    . "\t^" . "<" x ($cols-8) . "~~\n"
                    . '$entry' . "\n"
                    . ".\n";
           print $format if $Debugging;
           eval $format;
           die $@ if $@;

       Which would generate a format looking something like this:

        format STDOUT =
        ^<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<
        $entry
                ^<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<~~
        $entry
        .

       Here's a little program that's somewhat like fmt(1):

        format =
        ^<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<< ~~
        $_

        .

        $/ = '';
        while (<>) {
            s/\s*\n\s*/ /g;
            write;
        }

   Footers
       While $FORMAT_TOP_NAME  contains  the  name  of  the  current  header  format,  there  is  no
       corresponding mechanism to automatically do the same thing for a footer.  Not knowing how big
       a format is going to be until you evaluate it is one of the major problems.  It's on the TODO
       list.

       Here's  one  strategy:   If  you  have  a  fixed-size footer, you can get footers by checking
       $FORMAT_LINES_LEFT before each write() and print the footer yourself if necessary.

       Here's another strategy: Open a pipe to yourself, using "open(MYSELF, "|-")" (see  "open"  in
       perlfunc)  and  always  write() to MYSELF instead of STDOUT.  Have your child process massage
       its STDIN to rearrange headers and footers  however  you  like.   Not  very  convenient,  but
       doable.

   Accessing Formatting Internals
       For  low-level access to the formatting mechanism, you may use formline() and access $^A (the
       $ACCUMULATOR variable) directly.

       For example:

           $str = formline <<'END', 1,2,3;
           @<<<  @|||  @>>>
           END

           print "Wow, I just stored '$^A' in the accumulator!\n";

       Or to make an swrite() subroutine, which is to write() what  sprintf()  is  to  printf(),  do
       this:

           use Carp;
           sub swrite {
               croak "usage: swrite PICTURE ARGS" unless @_;
               my $format = shift;
               $^A = "";
               formline($format,@_);
               return $^A;
           }

           $string = swrite(<<'END', 1, 2, 3);
        Check me out
        @<<<  @|||  @>>>
        END
           print $string;

WARNINGS
       The  lone  dot  that  ends a format can also prematurely end a mail message passing through a
       misconfigured Internet mailer (and based on experience, such misconfiguration  is  the  rule,
       not  the  exception).  So when sending format code through mail, you should indent it so that
       the format-ending dot is not on the left margin; this will prevent SMTP cutoff.

       Lexical variables (declared with "my") are not visible within a format unless the  format  is
       declared within the scope of the lexical variable.

       If  a program's environment specifies an LC_NUMERIC locale and "use locale" is in effect when
       the format is declared, the locale  is  used  to  specify  the  decimal  point  character  in
       formatted  output.   Formatted  output  cannot be controlled by "use locale" at the time when
       write() is called. See perllocale for further discussion of locale handling.

       Within strings that are to be displayed in a fixed-length text field, each control  character
       is  substituted  by a space. (But remember the special meaning of "\r" when using fill mode.)
       This is done to avoid misalignment when control characters "disappear" on some output media.

perl v5.38.2                                 2026-08-18                                  PERLFORM(1)