Using a regular expression to replace upper case repeated letters in python with a single lowercase letter

Pass a function as the repl argument. The MatchObject is passed to this function and .group(1) gives the first parenthesized subgroup:

import re
s = 'start TT end'
callback = lambda pat: pat.group(1).lower()
re.sub(r'([A-Z]){2}', callback, s)

EDIT
And yes, you should use ([A-Z])\1 instead of ([A-Z]){2} in order to not match e.g. AZ. (See @bobince's answer.)

import re
s = 'start TT end'
re.sub(r'([A-Z])\1', lambda pat: pat.group(1).lower(), s) # Inline

Gives:

'start t end'

You can't change case in a replacement string. You would need a replacement function:

>>> def replacement(match):
...     return match.group(1).lower()
... 
>>> re.sub(r'([A-Z])\1', replacement, 'start TT end')
'start t end'

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