[elchief]

`x` mode and DEFINE can your friends. Example

  (?xi)
  (?(DEFINE)
  
    (?<year> \d{4} )  # I'm a comment
  
    (?<brand> honda|toyota )
  
    (?<model> crx|prius )
  )

...later...

  ((?&year)) ((?&brand)) ((?&model))