SRE does not always correctly handle groups in 
alternatives in repeats.  For example:

>>> re.match('((a)|b)*', 'abc').groups()
('b', '')

Group 2 should obviously never be an empty string.  As I 
understand it, the rule for groups inside a repeat is that 
they should have the last value they matched during the 
iterations of the repeat (or None if they never match), so 
in the above case Group 2 should be 'a'.  To fix this, it 
appears that (when inside a repeat) the BRANCH 
opcode must call mark_save before trying an alternative 
and then call mark_restore if the alternative fails.  The 
attached patch does this.