sub match {
  shift =~ m/
       (?:    [^(]*\(([^)]*?)\)    )    # Both parens.
    |  (?:    \((.*)$              )    # Left paren.
    |  (?:    ^(.*)\)              )    # Right paren.
    |  (?:    ^[^()]*()$           )    # No parens (empty capture).
  /x;
  return $^N // ();
}