How to find and replace nth occurrence of word in a sentence using python regular expression?

Use negative lookahead like below.

>>> s = "cat goose  mouse horse pig cat cow"
>>> re.sub(r'^((?:(?!cat).)*cat(?:(?!cat).)*)cat', r'\1Bull', s)
'cat goose  mouse horse pig Bull cow'

DEMO

• ^ Asserts that we are at the start.
• (?:(?!cat).)* Matches any character but not of cat , zero or more times.
• cat matches the first cat substring.
• (?:(?!cat).)* Matches any character but not of cat , zero or more times.
• Now, enclose all the patterns inside a capturing group like ((?:(?!cat).)*cat(?:(?!cat).)*), so that we could refer those captured chars on later.
• cat now the following second cat string is matched.

OR

>>> s = "cat goose  mouse horse pig cat cow"
>>> re.sub(r'^(.*?(cat.*?){1})cat', r'\1Bull', s)
'cat goose  mouse horse pig Bull cow'

Change the number inside the {} to replace the first or second or nth occurrence of the string cat

To replace the third occurrence of the string cat, put 2 inside the curly braces ..

>>> re.sub(r'^(.*?(cat.*?){2})cat', r'\1Bull', "cat goose  mouse horse pig cat foo cat cow")
'cat goose  mouse horse pig cat foo Bull cow'

Play with the above regex on here …