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Gurusamy Sarathy


re - Perl pragma to alter regular expression behaviour


    use re 'taint';
    ($x) = ($^X =~ /^(.*)$/s);     # $x is tainted here

    use re 'eval';
    /foo(?{ $foo = 1 })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(?{ $foo = 1 })bar/;   # disallowed (with or without -T switch)


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 (which may not be interpolated in the regex). That is normally disallowed, since it is a potential security risk. Note that this pragma is ignored when perl detects tainted data, i.e. evaluation is always disallowed with tainted data. See "(?{ code })" in perlre.

See "Pragmatic Modules" in perlmodlib.