sub match {
  shift =~ m/
    (?|
         (?:    [^(]*\(([^)]*?)\)    )    # Both parens.
      |  (?:    \((.*)$              )    # Left paren.
      |  (?:    ^(.*)\)              )    # Right paren.
      |  (?:    ^[^()]*()$           )    # No parens (empty capture).
    )
  /x;
  return $1 // ();
}