1. text = 'UPPER PYTHON, lower python, Mixed Python'
    3. def matchcase(word):
    4.     def replace(m):
    5.         text = m.group()
    6.         if text.isupper():
    7.             return word.upper()
    8.         elif text.islower():
    9.             return word.lower()
    10.         elif text[0].isupper():
    11.             return word.capitalize()
    12.         else:
    13.             return word
    15.     return replace
    18. print(re.sub('python', matchcase('snake'), text, flags=re.I))
    19. >>> UPPER SNAKE, lower snake, Mixed Snake