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re(3pm)                               Perl Programmers Reference Guide                               re(3pm)



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 '/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);          # Finer tuned debugging options.
           use re qw(Debug More);
           no re qw(Debug ALL);           # Turn of 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, even if the regular expression contains variable
       interpolation.  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.

   '/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 commaseparated
       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.

       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.

           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

           All Enable DUMP and all execute options. Equivalent to:

                 use re 'debug';

           MORE
           More
               Enable TRIEM and all execute compile and execute options.

       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.

   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, prefering 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.



perl v5.16.2                                     2012-10-25                                          re(3pm)