How do I match on optional conditions?

I'm trying to create a rule that matches on a filename with one or more optional conditions. For example, a file name needs to start with a word (abc) but might optionally have two words (abc [abc]). There have been other situations where indicating optional matches would make my rules more flexible.

Are you saying you would like a rule to run on a file that begins with abc but not run if the file begins with abc-abc? If so, you can just create a rule that runs if pattern 1 matches and pattern 2 does not match. So run if abc AND not abc-abc.