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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';                # 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 operations on tainted data aren't meant to extract safe substrings, but to perform other trans-formations. transformations.
       formations.

       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 "(?{ ... })" asser-tions. assertions.
       tions.

       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 inter-preter interpreter
       preter 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-sepa-rated comma-separated
       rated 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 v5.8.9                                      2001-09-21                                          re(3pm)