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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 val-
       ues 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 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 secu-
       rity 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 term-
       cap color sequences.  Set $ENV{PERL_RE_TC} to a comma-separated list of "termcap" properties to use for high-
       lighting strings on/off, pre-point part on/off.  See "Debugging regular expressions" in perldebug for addi-
       tional 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)