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 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.

       "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 /msx on by default, simply
       put

           use re '/msx';

       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'".

       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 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 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.

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

           ALL Enable all options at once except OFFSETS, OFFSETSDBG and BUFFERS.  (To get every
               single option without exception, use both ALL and EXTRA.)

           All Enable DUMP and all 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.

       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.

       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.

SEE ALSO

       "Pragmatic Modules" in perlmodlib.