Turns out, the previous regular expression does not handle fractional series numbers.
Such as:
Budung Kuleeeee - [Silly Horror 2.5] - Horror Horror Hor. epub
Also a fix so that the last letter of the title is not moved to Publisher in the case when the title is the very last part of the file name.
Here is the fixed one:
(?P<author>[^_-]+) -?\s*(\[|\()?\s*(?P<series>[^_0-9\[\(-]*)(?P<series_index>[0-9.]*)\s*(\]|\))?\s*-\s*(?P<title>[^_\[\(]+)\s*(?P<publisher>[^$]*)\s* ?
Still currently not supported:
when the name of the series itself contains a number followed by the series number:
Some Author - [Series Name Contains Number 21 02] - Title.rar
when the series name and numer are separated by a dash, so we have already three dashes:
Some Author - Some Series - 16 - Some Title.rar