The group must exist to reuse it. What should this regex do in your opinion. What about RISHMI\(T |IM|\)" or RISHM\(\(?:T |IM\)?\)"? Errors should never pass silently unless explicitly silenced \(PEP 20\). Maybe replacing empty strings could lead to unwanted side effects but I have'nt thought about it.

This regex here is just an example, and probably a bad one \(as the regex it does nothing by this replacement\). Your suggestion regarding the specific regex \(to use inner optional group within group\) would probably fix this specific regex, but this is workaround - replace.py should support replacing capturing optional capturing group the same way re.findall behaves.

The behaviour of replacing None to empty string is compatible with the behaviour of re.findall \(re.findall\('a\(b\)?\(c\)','ac'\) => \[\('', 'c'\)\]\) and with regex engines of most languages \(in JS: 'ac'.replace\(/a\(b\)?\(c\)/,'a$1c'\)\), though python re isn't consistent here \(re.sub\('a\(b\)?\(c\)','X\\\1','ac'\) - is error\).

I had my own fights with this problem and my conclusion was that there's nothing to do about it but rewriting your regexes, it's how python works.
Mostly, what's nasty is the idiotic error message.