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