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
    ...

    use re qw(Debug All);          # Finer tuned debugging options.
    use re qw(Debug More);         # Similarly not lexically scoped.
    no re qw(Debug ALL);           # Turn of all re dugging and unload the module.

(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 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 perlre.

For the purpose of this pragma, interpolation of precompiled 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.

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 perldebug for additional info.

Similarly use re 'Debug' produces debugging output, the difference being that it allows the fine tuning of what debugging output will be emitted. Following the 'Debug' keyword one of several options may be provided: COMPILE, EXECUTE, TRIE_COMPILE, TRIE_EXECUTE, TRIE_MORE, OPTIMISE, OFFSETS and ALL. Additionally the special keywords 'All' and 'More' may be provided. 'All' represents everything but OPTIMISE and OFFSETS and TRIE_MORE, and 'More' is similar but include TRIE_MORE. Saying no re Debug => 'EXECUTE' will disable executing debug statements and saying use re Debug => 'EXECUTE' will turn it on. Note that these flags can be set directly via ${^RE_DEBUG_FLAGS} by using the following flag values:

    RE_DEBUG_COMPILE       1
    RE_DEBUG_EXECUTE       2
    RE_DEBUG_TRIE_COMPILE  4
    RE_DEBUG_TRIE_EXECUTE  8
    RE_DEBUG_TRIE_MORE    16
    RE_DEBUG_OPTIMISE     32
    RE_DEBUG_OFFSETS      64

The directive use re 'debug' and its equivalents are not lexically scoped, as the other directives are. They have both compile-time and run-time effects.

See "Pragmatic Modules" in perlmodlib.