I tried adding parentheses around (?P<author>[^_]+) followed by a question mark, but that didn't seem to do the trick. You said I needed to add some titles for you, so here they are if you're willing to have a look.
These are all from the same author, so I want that to show up:
Beth Baker - FBI - 01 - The Games People Play
Beth Baker - FBI - 02 - The Devil’s Den
Beth Baker - FBI - 03 - Memories of the Dead
These are from various authors, about the same subject
Brothers - 01 - Reading of the Will
Brothers - 02 - Fortunate Sons
Brothers - 03 - A Matter Of Family
These two go well together:
(?P<series>.+) - (?P<series_index>\d+) - (?P<title>.*)
Brothers - 01 - Reading of the Will
If I change it to ((?P<author>[^_]+))? - (?P<series>.+) - (?P<series_index>\d+) - (?P<title>.*) it doesn't work. Did I place the parentheses and question mark in the proper place? I googled various phrases containing calibre, regex, optional author but was unable to find what I needed - all possibilities seemed very complicated and didn't achieve what I was looking for when I tried them (most of my searches pointed to this forum so I went through most of them).