re

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



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’;		  # NOT lexically scoped (as others are)
	   /^(.*)$/s;			  # output debugging info during
					  #	compile and run time

	   use re ’debugcolor’;		  # same as ’debug’, but with colored output
	   ...

       (We use $^X in these examples because it’s tainted by default.)

DESCRIPTION
       When "use re ’taint’" is in effect, and a tainted string is the target
       of a regex, the regex memories (or values returned by the m// operator
       in list context) are tainted.  This feature is useful when regex oper-
       ations on tainted data aren’t meant to extract safe substrings, but to
       perform other transformations.

       When "use re ’eval’" is in effect, a regex is allowed to contain "(?{
       ... })" zero-width assertions even if 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.

       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.

       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.

       The directive "use re ’debug’" is not lexically scoped, as the other
       directives are.	It has both compile-time and run-time effects.

       See "Pragmatic Modules" in perlmodlib.



perl v5.8.8			  2001-09-21				re(3)