Question

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 occurance of ‘_a’ to replace or last ‘.’

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

Answer 1

 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