


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


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 expresssions.  See
       the section on "(?{ code })" in the perlre manpage.

       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.



2001-05-30                 perl v5.6.1                          1





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


       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 the Debugging
       regular expressions entry in the perldebug manpage 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 the Pragmatic Modules entry in the perlmodlib manpage.






































2001-05-30                 perl v5.6.1                          2


