I think this is from Tom Christiansen's Perl Cookbook, but it's not handy right now... so here goes:

my $np;
$np = qr{
           \(
           (?:
              (?> [^(  )]+ )    # Non-capture group w/o backtracking
            |
              (??{ $np })     # Group with matching parens
           )*
           \)
        }x;
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I think you could also look at the Text::Balanced module's extract_bracketed subroutine to see how it's done there.

Match time pattern interpolation makes Perl's regexes much more powerful than most regex engines... not quite the power of a grammar/parser but still quite powerful, given their compactness. Of course, this is just an intuition on my part... is there a proof (or counterexample) that shows that regexes cannot in general do what a more general parser can do?