I had the same problem so I started using booksorter
to rename my files to
Author - Series # - Title.lit
then used this for the add to
(?P<author>[^_-]+) -?\s*(?P<series>[^_0-9-]*)(?P<series_index>[0-9]*)\s*-\s*(?P<title>[^_].+) ?
I did see somewhere on the forum the regex you are looking for but couldn't find it