a = 'мама мыла раму'

b = 'Мама, мыла Раму™'

>>> import re
>>> s1 = "Hello world! :)"
>>> s2 = "helloworld"
>>> re.sub("[^\w]","",s1.lower()) == re.sub("[^\w]","",s2.lower())
True

re.sub("[^a-z0-9]","",s1.lower()) == re.sub("[^a-z0-9]","",s2.lower())

>>> def f(s):
...     return tuple(i.lower() for i in s if i.isalpha())
... 
>>> a = 'мама мыла раму'
>>> b = 'Мама, мыла Раму™'
>>> f(a) == f(b)
True
>>> f('мама') == f('мыла')
False
>>>