our $pat;
    $pat = qr!
        (?:
            \[
            (?>
                (?>[^\[\]]+)
                |
                (??{$pat})
            )*
            \](?:\,\s)*
        )
    !x;
    
    if( $s =~ /\[$x1\,\s?($pat)*\[\[?$x2/ ) {
        return 1;
    }else{
        return 0;
    }