This example uses the supplied "null" pattern with
  the DECTPU alternation operator to include an optional
  element in a pattern.

  Supposing that, in the previous example, some of the
  disk names do not include the leading "U", for example
  DISK7. The following filter replaces disk names with or
  without the leading "U":

     global_replace ( ("U"|null) + "DISK" + number,
                      "DISK_NAME")