


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.



2000-11-02                 perl v5.6.0                          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.






































2000-11-02                 perl v5.6.0                          2


