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Rafaël Garcia-Suarez > perl-5.9.4 > re



Annotate this POD

Module Version: 0.0601   Source   Latest Release: perl-5.28.0


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

    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.)


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:


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. Options are divided into three groups, those related to compilation, those related to execution and those related to special purposes. The options are as follows:

Compile related options

Turns on all compile related debug options.


Turns on debug output related to the process of parsing the pattern.


Enables output related to the optimisation phase of compilation.


Detailed info about trie compilation.


Dump the final program out after it is compiled and optimised.


Dump offset information. This can be used to see how regops correlate to the pattern. Output format is


Where 1 is the position of the first char in the string. Note that position can be 0, or larger than the actual length of the pattern, likewise length can be zero.

Execute related options

Turns on all execute related debug options.


Turns on debugging of the main matching loop.


Extra debugging of how tries execute.


Enable debugging of start point optimisations.

Extra debugging options

Turns on all "extra" debugging options.


Enable enhanced TRIE debugging. Enhances both TRIE_EXECUTE and TRIE_COMPILE.


Enable debugging of offsets information. This emits copious amounts of trace information and doesnt mesh well with other debug options.

Almost definately only useful to people hacking on the offsets part of the debug engine.

Other useful flags

These are useful shortcuts to save on the typing.


Enable all compile and execute options at once.


Enable DUMP and all execute options. Equivelent to:

  use re 'debug';

Enable TRIE_MORE and all execute compile and execute options.

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.

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