replacing with keeping cases constant

replace2.py

def matchcase(word):
    def replace(m):
        text = m.group()
        if text.isupper():
            return word.upper()
        elif text.islower():
            return word.lower()
        elif text[0].isupper():
            return word.capitalize()
        else:
            return word
    return replace
    
text = "HELLO WORLD, hello world, Hello World"
re.sub("hello",matchcase('good morning'),text,flags=re.IGNORECASE)

# Output: 'GOOD MORNING WORLD, good morning world, Good morning World'