Current Perl documentation can be found at perldoc.perl.org.

Here is our local, out-dated (pre-5.6) version:

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, nor can they. For that 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 is the CPAN module Parse::RecDescent, the standard module Text::Balanced, the byacc program, and Mark-Jason Dominus's excellent py tool at http://www.plover.com/~mjd/perl/py/ .

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
    }