phpman > man > re(3perl)

NAME
    re - Perl pragma to alter regular expression behaviour

SYNOPSIS
        use re 'taint';
        ($x) = ($^X =~ /^(.*)$/s);     # $x is tainted here

        $pat = '(?{ $foo = 1 })';
        use re 'eval';
        /foo${pat}bar/;                # won't fail (when not under -T
                                       # switch)

        {
            no re 'taint';             # the default
            ($x) = ($^X =~ /^(.*)$/s); # $x is not tainted here

            no re 'eval';              # the default
            /foo${pat}bar/;            # disallowed (with or without -T
                                       # switch)
        }

        use re 'strict';               # Raise warnings for more conditions

        use re '/ix';
        "FOO" =~ / foo /; # /ix implied
        no re '/x';
        "FOO" =~ /foo/; # just /i implied

        use re 'debug';                # output debugging info during
        /^(.*)$/s;                     # compile and run time


        use re 'debugcolor';           # same as 'debug', but with colored
                                       # output
        ...

        use re qw(Debug All);          # Same as "use re 'debug'", but you
                                       # can use "Debug" with things other
                                       # than 'All'
        use re qw(Debug More);         # 'All' plus output more details
        no re qw(Debug ALL);           # Turn on (almost) all re debugging
                                       # in this scope

        use re qw(is_regexp regexp_pattern); # import utility functions
        my ($pat,$mods)=regexp_pattern(qr/foo/i);
        if (is_regexp($obj)) {
            print "Got regexp: ",
                scalar regexp_pattern($obj); # just as perl would stringify
        }                                    # it but no hassle with blessed
                                             # re's.

    (We use $^X in these examples because it's tainted by default.)

DESCRIPTION
  'taint' mode
    When "use re 'taint'" is in effect, and a tainted string is the target of a regexp, the regexp
    memories (or values returned by the m// operator in list context) are tainted. This feature is
    useful when regexp operations on tainted data aren't meant to extract safe substrings, but to
    perform other transformations.

  'eval' mode
    When "use re 'eval'" is in effect, a regexp is allowed to contain "(?{ ... })" zero-width
    assertions and "(??{ ... })" postponed subexpressions that are derived from variable
    interpolation, rather than appearing literally within the regexp. That is normally disallowed,
    since it is a potential security risk. Note that this pragma is ignored when the regular
    expression is obtained from tainted data, i.e. evaluation is always disallowed with tainted
    regular expressions. See "(?{ code })" in perlre and "(??{ code })" in perlre.

    For the purpose of this pragma, interpolation of precompiled regular expressions (i.e., the
    result of "qr//") is *not* considered variable interpolation. Thus:

        /foo${pat}bar/

    *is* allowed if $pat is a precompiled regular expression, even if $pat contains "(?{ ... })"
    assertions or "(??{ ... })" subexpressions.

  'strict' mode
    Note that this is an experimental feature which may be changed or removed in a future Perl
    release.

    When "use re 'strict'" is in effect, stricter checks are applied than otherwise when compiling
    regular expressions patterns. These may cause more warnings to be raised than otherwise, and
    more things to be fatal instead of just warnings. The purpose of this is to find and report at
    compile time some things, which may be legal, but have a reasonable possibility of not being the
    programmer's actual intent. This automatically turns on the "regexp" warnings category (if not
    already on) within its scope.

    As an example of something that is caught under ""strict'", but not otherwise, is the pattern

     qr/\xABC/

    The "\x" construct without curly braces should be followed by exactly two hex digits; this one
    is followed by three. This currently evaluates as equivalent to

     qr/\x{AB}C/

    that is, the character whose code point value is 0xAB, followed by the letter "C". But since "C"
    is a hex digit, there is a reasonable chance that the intent was

     qr/\x{ABC}/

    that is the single character at 0xABC. Under 'strict' it is an error to not follow "\x" with
    exactly two hex digits. When not under 'strict' a warning is generated if there is only one hex
    digit, and no warning is raised if there are more than two.

    It is expected that what exactly 'strict' does will evolve over time as we gain experience with
    it. This means that programs that compile under it in today's Perl may not compile, or may have
    more or fewer warnings, in future Perls. There is no backwards compatibility promises with
    regards to it. Also there are already proposals for an alternate syntax for enabling it. For
    these reasons, using it will raise a "experimental::re_strict" class warning, unless that
    category is turned off.

    Note that if a pattern compiled within 'strict' is recompiled, say by interpolating into another
    pattern, outside of 'strict', it is not checked again for strictness. This is because if it
    works under strict it must work under non-strict.

  '/flags' mode
    When "use re '/*flags*'" is specified, the given *flags* are automatically added to every
    regular expression till the end of the lexical scope. *flags* can be any combination of 'a',
    'aa', 'd', 'i', 'l', 'm', 'n', 'p', 's', 'u', 'x', and/or 'xx'.

    "no re '/*flags*'" will turn off the effect of "use re '/*flags*'" for the given flags.

    For example, if you want all your regular expressions to have /msxx on by default, simply put

        use re '/msxx';

    at the top of your code.

    The character set "/adul" flags cancel each other out. So, in this example,

        use re "/u";
        "ss" =~ /\xdf/;
        use re "/d";
        "ss" =~ /\xdf/;

    the second "use re" does an implicit "no re '/u'".

    Similarly,

        use re "/xx";   # Doubled-x
        ...
        use re "/x";    # Single x from here on
        ...

    Turning on one of the character set flags with "use re" takes precedence over the "locale"
    pragma and the 'unicode_strings' "feature", for regular expressions. Turning off one of these
    flags when it is active reverts to the behaviour specified by whatever other pragmata are in
    scope. For example:

        use feature "unicode_strings";
        no re "/u"; # does nothing
        use re "/l";
        no re "/l"; # reverts to unicode_strings behaviour

  'debug' mode
    When "use re 'debug'" is in effect, perl emits debugging messages when compiling and using
    regular expressions. The output is the same as that obtained by running a "-DDEBUGGING"-enabled
    perl interpreter with the -Dr switch. It may be quite voluminous depending on the complexity of
    the match. Using "debugcolor" instead of "debug" enables a form of output that can be used to
    get a colorful display on terminals that understand termcap color sequences. Set
    $ENV{PERL_RE_TC} to a comma-separated list of "termcap" properties to use for highlighting
    strings on/off, pre-point part on/off. See "Debugging Regular Expressions" in perldebug for
    additional info.

    As of 5.9.5 the directive "use re 'debug'" and its equivalents are lexically scoped, as the
    other directives are. However they have both compile-time and run-time effects.

    See "Pragmatic Modules" in perlmodlib.

  'Debug' mode
    Similarly "use re 'Debug'" produces debugging output, the difference being that it allows the
    fine tuning of what debugging output will be emitted. Options are divided into three groups,
    those related to compilation, those related to execution and those related to special purposes.
    The options are as follows:

    Compile related options

        COMPILE
            Turns on all non-extra compile related debug options.

        PARSE
            Turns on debug output related to the process of parsing the pattern.

        OPTIMISE
            Enables output related to the optimisation phase of compilation.

        TRIEC
            Detailed info about trie compilation.

        DUMP
            Dump the final program out after it is compiled and optimised.

        FLAGS
            Dump the flags associated with the program

        TEST
            Print output intended for testing the internals of the compile process

    Execute related options

        EXECUTE
            Turns on all non-extra execute related debug options.

        MATCH
            Turns on debugging of the main matching loop.

        TRIEE
            Extra debugging of how tries execute.

        INTUIT
            Enable debugging of start-point optimisations.

    Extra debugging options

        EXTRA
            Turns on all "extra" debugging options.

        BUFFERS
            Enable debugging the capture group storage during match. Warning, this can potentially
            produce extremely large output.

        TRIEM
            Enable enhanced TRIE debugging. Enhances both TRIEE and TRIEC.

        STATE
            Enable debugging of states in the engine.

        STACK
            Enable debugging of the recursion stack in the engine. Enabling or disabling this option
            automatically does the same for debugging states as well. This output from this can be
            quite large.

        GPOS
            Enable debugging of the \G modifier.

        OPTIMISEM
            Enable enhanced optimisation debugging and start-point optimisations. Probably not
            useful except when debugging the regexp engine itself.

        OFFSETS
            Dump offset information. This can be used to see how regops correlate to the pattern.
            Output format is

               NODENUM:POSITION[LENGTH]

            Where 1 is the position of the first char in the string. Note that position can be 0, or
            larger than the actual length of the pattern, likewise length can be zero.

        OFFSETSDBG
            Enable debugging of offsets information. This emits copious amounts of trace information
            and doesn't mesh well with other debug options.

            Almost definitely only useful to people hacking on the offsets part of the debug engine.

        DUMP_PRE_OPTIMIZE
            Enable the dumping of the compiled pattern before the optimization phase.

        WILDCARD
            When Perl encounters a wildcard subpattern, (see "Wildcards in Property Values" in
            perlunicode), it suspends compilation of the main pattern, compiles the subpattern, and
            then matches that against all legal possibilities to determine the actual code points
            the subpattern matches. After that it adds these to the main pattern, and continues its
            compilation.

            You may very well want to see how your subpattern gets compiled, but it is likely of
            less use to you to see how Perl matches that against all the legal possibilities, as
            that is under control of Perl, not you. Therefore, the debugging information of the
            compilation portion is as specified by the other options, but the debugging output of
            the matching portion is normally suppressed.

            You can use the WILDCARD option to enable the debugging output of this subpattern
            matching. Careful! This can lead to voluminous outputs, and it may not make much sense
            to you what and why Perl is doing what it is. But it may be helpful to you to see why
            things aren't going the way you expect.

            Note that this option alone doesn't cause any debugging information to be output. What
            it does is stop the normal suppression of execution-related debugging information during
            the matching portion of the compilation of wildcards. You also have to specify which
            execution debugging information you want, such as by also including the EXECUTE option.

    Other useful flags
        These are useful shortcuts to save on the typing.

        ALL Enable all options at once except OFFSETS, OFFSETSDBG, BUFFERS, WILDCARD, and
            DUMP_PRE_OPTIMIZE. (To get every single option without exception, use both ALL and
            EXTRA, or starting in 5.30 on a "-DDEBUGGING"-enabled perl interpreter, use the -Drv
            command-line switches.)

        All Enable DUMP and all non-extra execute options. Equivalent to:

              use re 'debug';

        MORE
        More
            Enable the options enabled by "All", plus STATE, TRIEC, and TRIEM.

    As of 5.9.5 the directive "use re 'debug'" and its equivalents are lexically scoped, as are the
    other directives. However they have both compile-time and run-time effects.

  Exportable Functions
    As of perl 5.9.5 're' debug contains a number of utility functions that may be optionally
    exported into the caller's namespace. They are listed below.

    is_regexp($ref)
        Returns true if the argument is a compiled regular expression as returned by "qr//", false
        if it is not.

        This function will not be confused by overloading or blessing. In internals terms, this
        extracts the regexp pointer out of the PERL_MAGIC_qr structure so it cannot be fooled.

    regexp_pattern($ref)
        If the argument is a compiled regular expression as returned by "qr//", then this function
        returns the pattern.

        In list context it returns a two element list, the first element containing the pattern and
        the second containing the modifiers used when the pattern was compiled.

          my ($pat, $mods) = regexp_pattern($ref);

        In scalar context it returns the same as perl would when stringifying a raw "qr//" with the
        same pattern inside. If the argument is not a compiled reference then this routine returns
        false but defined in scalar context, and the empty list in list context. Thus the following

            if (regexp_pattern($ref) eq '(?^i:foo)')

        will be warning free regardless of what $ref actually is.

        Like "is_regexp" this function will not be confused by overloading or blessing of the
        object.

    regname($name,$all)
        Returns the contents of a named buffer of the last successful match. If $all is true, then
        returns an array ref containing one entry per buffer, otherwise returns the first defined
        buffer.

    regnames($all)
        Returns a list of all of the named buffers defined in the last successful match. If $all is
        true, then it returns all names defined, if not it returns only names which were involved in
        the match.

    regnames_count()
        Returns the number of distinct names defined in the pattern used for the last successful
        match.

        Note: this result is always the actual number of distinct named buffers defined, it may not
        actually match that which is returned by "regnames()" and related routines when those
        routines have not been called with the $all parameter set.

    regmust($ref)
        If the argument is a compiled regular expression as returned by "qr//", then this function
        returns what the optimiser considers to be the longest anchored fixed string and longest
        floating fixed string in the pattern.

        A *fixed string* is defined as being a substring that must appear for the pattern to match.
        An *anchored fixed string* is a fixed string that must appear at a particular offset from
        the beginning of the match. A *floating fixed string* is defined as a fixed string that can
        appear at any point in a range of positions relative to the start of the match. For example,

            my $qr = qr/here .* there/x;
            my ($anchored, $floating) = regmust($qr);
            print "anchored:'$anchored'\nfloating:'$floating'\n";

        results in

            anchored:'here'
            floating:'there'

        Because the "here" is before the ".*" in the pattern, its position can be determined
        exactly. That's not true, however, for the "there"; it could appear at any point after where
        the anchored string appeared. Perl uses both for its optimisations, preferring the longer,
        or, if they are equal, the floating.

        NOTE: This may not necessarily be the definitive longest anchored and floating string. This
        will be what the optimiser of the Perl that you are using thinks is the longest. If you
        believe that the result is wrong please report it via the perlbug utility.

    optimization($ref)
        If the argument is a compiled regular expression as returned by "qr//", then this function
        returns a hashref of the optimization information discovered at compile time, so we can
        write tests around it. If any other argument is given, returns "undef".

        The hash contents are expected to change from time to time as we develop new ways to
        optimize - no assumption of stability should be made, not even between minor versions of
        perl.

        For the current version, the hash will have the following contents:

        minlen
            An integer, the least number of characters in any string that can match.

        minlenret
            An integer, the least number of characters that can be in $& after a match. (Consider eg
            " /ns(?=\d)/ ".)

        gofs
            An integer, the number of characters before "pos()" to start match at.

        noscan
            A boolean, "TRUE" to indicate that any anchored/floating substrings found should not be
            used. (CHECKME: apparently this is set for an anchored pattern with no floating
            substring, but never used.)

        isall
            A boolean, "TRUE" to indicate that the optimizer information is all that the regular
            expression contains, and thus one does not need to enter the regexp runtime engine at
            all.

        anchor SBOL
            A boolean, "TRUE" if the pattern is anchored to start of string.

        anchor MBOL
            A boolean, "TRUE" if the pattern is anchored to any start of line within the string.

        anchor GPOS
            A boolean, "TRUE" if the pattern is anchored to the end of the previous match.

        skip
            A boolean, "TRUE" if the start class can match only the first of a run.

        implicit
            A boolean, "TRUE" if a "/.*/" has been turned implicitly into a "/^.*/".

        anchored/floating
            A byte string representing an anchored or floating substring respectively that any match
            must contain, or undef if no such substring was found, or if the substring would require
            utf8 to represent.

        anchored utf8/floating utf8
            A utf8 string representing an anchored or floating substring respectively that any match
            must contain, or undef if no such substring was found, or if the substring contains only
            7-bit ASCII characters.

        anchored min offset/floating min offset
            An integer, the first offset in characters from a match location at which we should look
            for the corresponding substring.

        anchored max offset/floating max offset
            An integer, the last offset in characters from a match location at which we should look
            for the corresponding substring.

            Ignored for anchored, so may be 0 or same as min.

        anchored end shift/floating end shift
            FIXME: not sure what this is, something to do with lookbehind. regcomp.c says: When the
            final pattern is compiled and the data is moved from the scan_data_t structure into the
            regexp structure the information about lookbehind is factored in, with the information
            that would have been lost precalculated in the end_shift field for the associated
            string.

        checking
            A constant string, one of "anchored", "floating" or "none" to indicate which substring
            (if any) should be checked for first.

        stclass
            A string representation of a character class ("start class") that must be the first
            character of any match.

            TODO: explain the representations.

SEE ALSO
    "Pragmatic Modules" in perlmodlib.