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       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 '/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.)


   '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:


       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

           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
               Turns on all compile related debug options.

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

               Enables output related to the optimisation phase of compilation.

               Detailed info about trie compilation.

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

       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 large.

               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

       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';

               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.

           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

           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

           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, 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.

           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 match.

           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.