


lib::re(3p)      Perl Programmers Reference Guide     lib::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
       perform other transformations.

       When use re 'eval' is in effect, a regex is allowed to
       contain (?{ ... }) zero-width assertions even if the regex
       contains variable interpolation.  This 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 the
       section on (?{ code }) in the perlre manpage.

       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.



22/Oct/1999            perl 5.005, patch 03                     1





lib::re(3p)      Perl Programmers Reference Guide     lib::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 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 the section on Debugging regular
       expressions 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 section on Pragmatic Modules in the perlmodlib
       manpage.






































22/Oct/1999            perl 5.005, patch 03                     2


