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 default.)


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 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 (?{ 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:


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 properties to use for highlighting strings on/off, pre-point part on/off. See 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 Pragmatic Modules in the perlmodlib manpage.