NAME
    re::engine::RE2 - RE2 regex engine

SYNOPSIS
        use re::engine::RE2;

        if ("Hello, world" =~ /Hello, (world)/) {
            print "Greetings, $1!";
        }

DESCRIPTION
    This module replaces perl's regex engine in a given lexical scope with
    RE2.

    RE2 is a primarily DFA based regexp engine from Google that is very fast
    at matching large amounts of text. However it does not support look
    behind and some other Perl regular expression features. See RE2's
    website <http://code.google.com/p/re2> for more information.

    Fallback to normal Perl regexp is implemented by this module. If RE2 is
    unable to compile a regexp it will use Perl instead, therefore features
    not implemented by RE2 don't suddenly stop working, they will just use
    Perl's regexp implementation.

METHODS
    To access extra functionality of RE2 methods can be called on a compiled
    regular expression (i.e. a "qr//").

    *   "possible_match_range([length = 10])"

        Returns an array of two strings: where the expression will start
        matching and just after where it will finish matching. See RE2's
        documentation on PossibleMatchRange for further details.

        Example:

            my($min, $max) = qr/^(a|b)/->possible_match_range;
            is $min, 'a';
            is $max, 'c';'

    *   "named_captures()"

        Returns a hash of the name captures and index.

        Example:

            my $named_captures = qr/(?P<a>\w+) (?P<d>\w+)/->named_captures;
            is $named_captures->{a}, 1;
            is $named_captures->{d}, 2;

    *   "number_of_capture_groups()"

        Return number of capture groups

        Example:

            my $captures = qr/(Hello), (world)/->number_of_capture_groups;
            is $captures, 2;

PRAGMA OPTIONS
    Various options can be set by providing options to the "use" line. These
    will be pragma scoped.

    *   "-max_mem => 1<<24"

        Configure RE2's memory limit.

    *   "-strict => 1"

        Be strict, i.e. don't allow regexps that are not supported by RE2.

    *   "-longest_match => 1"

        Match on the longest match in alternations. For example with this
        option set matching "abc" against "(a|abc)" will match "abc",
        without depending on order.

    *   "-never_nl => 1"

        Never match a newline ("\n") even if the provided regexp contains
        it.

PERFORMANCE
    Performance is really the primary reason for using RE2, so here's some
    benchmarks. Like any benchmark take them with a pinch of salt.

  Simple matching
      my $foo = "foo bar baz";
      $foo =~ /foo/;
      $foo =~ /foox/;

    On this very simple match RE2 is actually slower:

               Rate  re2   re
      re2  674634/s   -- -76%
      re  2765739/s 310%   --

  URL matching
    Matching "m{([a-zA-Z][a-zA-Z0-9]*)://([^ /]+)(/[^ ]*)?|([^ @]+)@([^
    @]+)}" against a several KB file:

            Rate    re   re2
      re  35.2/s    --  -99%
      re2 2511/s 7037%    --

  Many alternatives
    Matching a string against a regexp with 17,576 alternatives ("aaa ..
    zzz").

    This uses trie matching on Perl (obviously RE2 does similar by default).

      $ perl misc/altern.pl
              Rate   re  re2
      re   52631/s   -- -91%
      re2 554938/s 954%   --

NOTES
    *   No support for "m//x"

        The "/x" modifier is not supported. (There's no particular reason
        for this, just RE2 itself doesn't support it). Fallback to Perl
        regexp will happen automatically if "//x" is used.

    *   "re2/dfa.cc:447: DFA out of memory: prog size xxx mem yyy"

        If you attempt to compile a really large regular expression you may
        get this error. RE2 has an internal limit on memory consumption for
        the DFA state tables. By default this is 8 MiB.

        If you need to increase this size then use the max_mem parameter:

          use re::engine::RE2 -max_mem => 8<<23; # 64MiB

    *   How do I tell if RE2 will be used?

        See if your regexp is matching quickly or slowly ;).

        Alternatively normal OO concepts apply and you may examine the
        object returned by "qr//":

          use re::engine::RE2;

          ok qr/foo/->isa("re::engine::RE2");

          # Perl Regexp used instead
          ok not qr/(?<=foo)bar/->isa("re::engine::RE2");

        If you wish to force RE2, use the "-strict" option.

BUGS
    Known issues:

    *   Unicode handling

        Currently the Unicode handling of re::engine::RE2 does not fully
        match Perl's behaviour.

        The UTF-8 flag of the regexp currently determines how the string is
        matched. This is obviously broken, so will be fixed at some point.

    *   Final newline matching differs to Perl

          "\n" =~ /$/

        The above is true in Perl, false in RE2. To work around the issue
        you can write "\n?\z" when you mean Perl's "$".

    Please report bugs or provide patches at
    <https://github.com/dgl/re-engine-RE2>.

AUTHORS
    David Leadbeater <dgl[at]dgl[dot]cx>

COPYRIGHT
    Copyright 2010 David Leadbeater.

    Based on re::engine::PCRE:

    Copyright 2007 Ævar Arnfjörð Bjarmason.

    The original version was copyright 2006 Audrey Tang <cpan@audreyt.org>
    and Yves Orton.

    This program is free software; you can redistribute it and/or modify it
    under the same terms as Perl itself.

    (However the bundled copy of RE2 has a different copyright owner and is
    under a BSD-like license, see re2/LICENSE.)