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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 '/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.

   '/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

	   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

	       Enables output related to the optimisation phase of

	       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

	       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.)

	   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, 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

	   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 v5.18.2			  2014-01-06			     re(3perl)

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