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

Question

Using python regular expression only, how to find and replace nth occurrence of word in a sentence? For example:

str = \'cat goose  mouse horse pig cat cow\'         


        

Answer 1

I would define a function that will work for every regex:

import re

def replace_ith_instance(string, pattern, new_str, i = None, pattern_flags = 0):
    # If i is None - replacing last occurrence
    match_obj = re.finditer(r'{0}'.format(pattern), string, flags = pattern_flags)
    matches = [item for item in match_obj]
    if i == None:
        i = len(matches)
    if len(matches) == 0 or len(matches) < i:
        return string
    match = matches[i - 1]
    match_start_index = match.start()
    match_len = len(match.group())

    return '{0}{1}{2}'.format(string[0:match_start_index], new_str, string[match_start_index + match_len:])

A working example:

str = 'cat goose  mouse horse pig cat cow'
ns = replace_ith_instance(str, 'cat', 'Bull', 2)
print(ns)

The output:

cat goose  mouse horse pig Bull cow

Another example:

str2 = 'abc abc def abc abc'
ns = replace_ith_instance(str2, 'abc\s*abc', '666')
print(ns)

The output:

abc abc def 666