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

       re - Perl pragma to alter regular expression behaviour

           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.)

   '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:


       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


       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


       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


       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

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

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


           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
               Turns on all compile related debug options.

               Turns on debug output related to the process of parsing the

               Enables output related to the optimisation phase of

               Detailed info about trie compilation.

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

               Dump the flags associated with the program

               Print output intended for testing the internals of the compile

       Execute related options
               Turns on all execute related debug options.

               Turns on debugging of the main matching loop.

               Extra debugging of how tries execute.

               Enable debugging of start-point optimisations.

       Extra debugging options
               Turns on all "extra" debugging options.

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

               Enable enhanced TRIE debugging. Enhances both TRIEE and TRIEC.

               Enable debugging of states in the engine.

               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

               Enable debugging of the \G modifier.

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

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


               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.

               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, or starting in 5.30 on a
               "-DDEBUGGING"-enabled perl interpreter, use the -Drv command-
               line switches.)

           All Enable DUMP and all execute options. Equivalent to:

                 use re 'debug';

               Enable the options enabled by "All", plus STATE, TRIEC, and

       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

           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.

           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.

           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


           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.

           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.

           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

           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.

       "Pragmatic Modules" in perlmodlib.

perl v5.30.0                      2023-11-23                         re(3perl)
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