Yes, append that group with a question mark.
Code:
(?P<series>[A-Za-z ]+)-(?P<series_index>[0-9]+) (?P<title>.+) \((?P<author>[A-Za-z. ]+)\) (?P<published>[0-9]+)?
If I had known the published date was supposed to be optional, I would've done that in the first place.
You get what you asked for. 
If I didn't have short-term memory loss and forget part of the OP...