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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 '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.

   '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.

       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 buffer 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 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-xsm: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 v5.12.1				    2010-07-01					  re(3pm)
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