import re
s = (
    """Mr. Smith bought cheapsite.com for 1.5 million dollars, i.e. he paid a lot for it. Did he mind? Adam Jones Jr. thinks he didn't. In any case, this isn't true... Well, with a probability of .9 it isn't."""
    )
m = re.split('(?<=[^A-Z].[.?]) +(?=[A-Z])', s)
for i in m:
    pass
Python — RegEx для разбиения текста на предложения (маркировка предложений)