regex - How to replace the Nth appearance of a needle in a haystack? (Python)

Question

I am trying to replace the Nth appearance of a needle in a haystack. I want to do this simply via re.sub(), but cannot seem to come up with an appropriate regex to solve this. I am trying to adapt: http://docstore.mik.ua/orelly/perl/cookbook/ch06_06.htm but am failing at spanning multilines, I suppose.

My current method is an iterative approach that finds the position of each occurrence from the beginning after each mutation. This is pretty inefficient and I would like to get some input. Thanks!

Answer 1

I think you mean re.sub. You could pass a function and keep track of how often it was called so far:

def replaceNthWith(n, replacement):
    def replace(match, c=[0]):
        c[0] += 1
        return replacement if c[0] == n else match.group(0)
    return replace

Usage:

re.sub(pattern, replaceNthWith(n, replacement), str)

But this approach feels a bit hacky, maybe there are more elegant ways.

DEMO