SRE is broken in some subtle ways when you combine 
capturing groups with assertions.  For example:

>>> re.match('((?!(a)c)[ab])*', 'abc').groups()
('b', '')

In the above '(a)' has matched an empty string.  Or 
worse:

>>> re.match('(a)((?!(b)*))*', 'abb').groups()
('b', None, None)

Here '(a)' matches 'b'.

Although Perl reports matches for groups in negative 
assertions, I think it is better to adopt the PCRE rule 
that these groups are always reported as unmatched 
outside the assertion (inside the assertion, if used with 
backreferences, they should behave as normal).  This 
would make the handling of subpatterns in negative 
assertions consistent with that of subpatterns in 
branches:

>>> re.match('(a)c|ab', 'ab').groups()
(None,)

In the above, although '(a)' matches before the branch 
fails, the failure of the branch means '(a)' is considered 
not to have matched.

Anyway, the attached patch is an effort to fix this 
problem by saving the values of marks before calling the 
assertion, and then restoring them afterwards (thus 
undoing whatever might have been done in the assertion).