[Python] How to replace only part of the match with python re.sub

I need to match two cases by one reg expression and do replacement

'long.file.name.jpg' -> 'long.file.name_suff.jpg'

'long.file.name_a.jpg' -> 'long.file.name_suff.jpg'

I'm trying to do the following

re.sub('(\_a)?\.[^\.]*$' , '_suff.',"long.file.name.jpg")


But this is cut the extension '.jpg' and I'm getting

long.file.name_suff. instead of long.file.name_suff.jpg I understand that this is because of [^.]*$ part, but I can't exclude it, because I have to find last occurrence of '_a' to replace or last '.'

Is there a way to replace only part of the match?

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