MirBSD manpage: re(3p)

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


     re - Perl pragma to alter regular expression behaviour


         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


     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 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 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 con-
     sidered variable interpolation.  Thus:


perl v5.8.8                2005-02-05                           1

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

     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 termcap color sequences.  Set
     $ENV{PERL_RE_TC} to a comma-separated list of "termcap" pro-
     perties to use for highlighting strings on/off, pre-point
     part on/off. See "Debugging regular expressions" in perlde-
     bug 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                2005-02-05                           2

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