$rxNest = qr{(?x)
    \(
    (?: (?>[^()]+) | (??{$rxNest}) )*
    \)
  };
  $string =~ /$rxNest/;

##</code><code>##

  $rxNest = qr{
    \(
    (?: (?>[^()]+) | (??{$rxNest}) )*
    \)
  }x;

##</code><code>##

  (?{ local @memoList = (@memoList, $+) })

##</code><code>##

  (?{ local $offset = $offset + 1; local $memoList[$offset] = $+ })

##</code><code>##

  $string =~ /^$rxNest\z/;