#!/usr/bin/perl

use strict;
use warnings;

my $string = qq~['X', ['S', ['Y', ['X0']], ['NP', ['YP', ['ZP']], 'VP'], 'DA']]~;

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

if( $string =~ /\[\'S\'\,\s($pat)*\[\'NP\'/ ) {
    print "matcht $1!\n";
    print "alles: $&\n";
}else{
    print "nix gefunden  :-(\n";
}