re.sub replaces matches. The replacement can be a string with \1 or a function of the match.

Goal

Mask digits in a phone line and rewrite totals.

import re
print(re.sub(r"\d", "#", "call 0712345678"))

import re
print(re.sub(r"(\d{4})-(\d{3})-(\d{3})", r"\1***", "0712-345-678"))

import re
def double(m):
    return str(int(m.group()) * 2)
print(re.sub(r"\d+", double, "Nairobi 12 Mombasa 9"))

import re
print(re.sub(r"\s+", ",", "Nairobi  12   10.5", count=2))