nth substring multiplicity using itertools

I am trying to write a function that returns a string with the nth substring replaced by a new substring.

I tried the following:

import re
from itertools import count

text = "Hello_Dear_Today_is_Nice_Today_is_Nice"

def replace_nth(text, sub, rep_sub, n):
    c = count(0)
    res = re.sub(r"{}".format(sub), lambda x: rep_sub if next(c) == n else x.group(), text)
    return res

print(replace_nth(text, "Today", "Today_2", 2))

But the returned string is the same, what am I doing wrong?

I am expecting:

Hello_Dear_Today_is_Nice_Today_2_is_Nice as a result

Without any deps, certainly a lot faster than re/itertools. If the string contains the substring n times, the number resulting from the split must be n+1 . If that is not the case, the function returns None

def replace_nth(text, sub, rep_sub, n):
    return (sub.join(g[:n]) + rep_sub + g[n]) if len(g:=text.split(sub,maxsplit=n))==n+1 else None

text = "Hello_Dear_Today_is_Nice_Today_is_Nice"

print(replace_nth(text, "Today", "Today_2", 2))