How do I write a regex to replace a word but keep its case in Python?

Question

Is this even possible?

Basically, I want to turn these two calls to sub into a single call:

re.sub(r'\bAword\b', 'Bword', mystring)
re.sub(r'\baword\b', 'bword', mystring)

What I'd really like is some sort of conditional substitution notation like:

re.sub(r'\b([Aa])word\b', '(?1=A:B,a:b)word')

I only care about the capitalization of the first character. None of the others.

Answer 1

You can have functions to parse every match:

>>> def f(match):
        return chr(ord(match.group(0)[0]) + 1) + match.group(0)[1:]

>>> re.sub(r'\b[aA]word\b', f, 'aword Aword')
'bword Bword'

Answer 2

OK, here's the solution I came up with, thanks to the suggestions to use a replace function.

re.sub(r'\b[Aa]word\b', lambda x: ('B' if x.group()[0].isupper() else 'b') + 'word', 'Aword  aword.')