#!/usr/bin/perl

use strict;
use warnings;

my $string = qq~['S', ['NP', ['XP', ['YP', ['ZP'], ['ZP']]] 'VP']]~;

our $pat;
$pat = qr!\[(?>(?>[^\[\]]+)|(??{$pat}))*\]!;

if( $string =~ /^($pat)$/ ){
    print "matcht $1!\n";
}