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 operations on tainted
       data aren't meant to extract safe substrings, but to per-
       form 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 nor-
       mally disallowed, since it is a potential security risk.
       Note that this pragma is ignored when the regular expres-
       sion 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 precom-
       piled 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 com-
       plexity 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 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)