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

 re.sub(r'(?:_a)?\.([^.]*)$', r'_suff.\1', "long.file.name.jpg")

?: starts a non matching group (SO answer), so (?:_a) is matching the _a but not enumerating it, the following question mark makes it optional.

So in English, this says, match the ending .<anything> that follows (or doesn't) the pattern _a

Another way to do this would be to use a lookbehind (see here). Mentioning this because they're super useful, but I didn't know of them for 15 years of doing REs

Just put the expression for the extension into a group, capture it and reference the match in the replacement:

re.sub(r'(?:_a)?(\.[^\.]*)$' , r'_suff\1',"long.file.name.jpg")

Additionally, using the non-capturing group (?:…) will prevent re to store to much unneeded information.

Put a capture group around the part that you want to preserve, and then include a reference to that capture group within your replacement text.

re.sub(r'(\_a)?\.([^\.]*)$' , r'_suff.\2',"long.file.name.jpg")