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In thinking further, I realized that positive assertions are also 
affected by the second problem.  E.g.:

>>> re.match('(a)(?:(?=(b)*)c)*', 'abb').groups()
('b', None)

The problem here is that a successful match in an assertion 
can leave marks at the top of the mark stack which then get 
popped in the wrong place.  Attaching a new patch which 
should catch this problem for both kinds of assertions (and 
which also should "unmark" groups in negative assertions).