replacement.py

#!/usr/bin/env python
# -*- coding: utf-8 -*-

import re

def simple_replace(s):
    return s.replace('A', 'a').replace('B', 'b').replace('C', 'c')

def dynamic_replace(s):
    for t in 'ABC':
        s = s.replace(t, t.lower())
    return s

upper_re = re.compile('[ABC]')
def re_replace(s):
    global upper_re
    return upper_re.sub(lambda m: m.group().lower(), s)

if __name__ == '__main__':

    s = 'aAabBbcCc'

    print simple_replace(s)
    print dynamic_replace(s)
    print re_replace(s)

    from timeit import timeit

    print timeit(lambda: simple_replace(s))
    # -> 1.07289290428

    print timeit(lambda: dynamic_replace(s))
    # -> 1.91307997704

    print timeit(lambda: re_replace(s))
    # -> 4.73019194603