import timeit
import re

not_characters = re.compile("[^\w]+") # _ and digits are considered to be characters
undescore_and_digits = re.compile("[_\d]+")


test_str = "qwe! rty - uio? pas!!! dfg 123123 __ @#$$#@::;;"

def regexp_replaces(s):
    only_chars = not_characters.sub("", s)
    return undescore_and_digits.sub("", only_chars)

def comprehension(s):
    return "".join([c.lower() for c in s if str.isalpha(c)])

print(regexp_replaces(test_str) == comprehension(test_str))

replace_timer = timeit.Timer("regexp_replaces(test_str)",
                             "from __main__ import regexp_replaces, test_str")
comprehension_timer = timeit.Timer("comprehension(test_str)",
                                   "from __main__ import comprehension, test_str")

print("Replaces       : %f" % replace_timer.timeit(number=1000000))
print("Comprehensions : %f" % comprehension_timer.timeit(number=1000000))