FAQ 4.23: How do I find matching/nesting anything?

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4.23: How do I find matching/nesting anything?

    This isn't something that can be done in one regular expression, no
    matter how complicated. To find something between two single characters,
    a pattern like "/x([^x]*)x/" will get the intervening bits in $1. For
    multiple ones, then something more like "/alpha(.*?)omega/" would be
    needed. But none of these deals with nested patterns. For balanced
    expressions using "(", "{", "[" or "<" as delimiters, use the CPAN
    module Regexp::Common, or see "(??{ code })" in perlre. For other cases,
    you'll have to write a parser.

    If you are serious about writing a parser, there are a number of modules
    or oddities that will make your life a lot easier. There are the CPAN
    modules Parse::RecDescent, Parse::Yapp, and Text::Balanced; and the
    byacc program. Starting from perl 5.8 the Text::Balanced is part of the
    standard distribution.

    One simple destructive, inside-out approach that you might try is to
    pull out the smallest nesting parts one at a time:

        while (s/BEGIN((?:(?!BEGIN)(?!END).)*)END//gs) {
            # do something with $1

    A more complicated and sneaky approach is to make Perl's regular
    expression engine do it for you. This is courtesy Dean Inada, and rather
    has the nature of an Obfuscated Perl Contest entry, but it really does

        # $_ contains the string to parse
        # BEGIN and END are the opening and closing markers for the
        # nested text.

        @( = ('(','');
        @) = (')','');
        @$ = (eval,$@!~/unmatched/i);
        print join("\n",@$[0..$#$]) if( $$[-1] );


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