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Gurusamy Sarathy


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';
    /^(.*)$/s;                     # output debugging info 
                                   # during compile and run time

(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 expresssions. See "(?{ code })" in perlre.

For the purpose of this pragma, interpolation of preexisting regular expressions is not considered a variable interpolation, thus


is allowed if $pat is a preexisting regular expressions, 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. See "Debugging regular expressions" in perldebug for additional info.

The directive use re 'debug' is not lexically scoped. It has both compile-time and run-time effects.

See "Pragmatic Modules" in perlmodlib.