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In certain cases a zero-width /Z match that should be replaced isn't.
An example might help:
 re.compile('(?m)(?P<trailing_ws>[ \t]+\r*$)|(?P<no_final_newline>(?<=[^\n])\Z)').subn(lambda m:next('<'+k+'>' for k,v in m.groupdict().items() if v is not None), 'foobar ')
this gives
 ('foobar<trailing_ws>', 1)
I would have expected
('foobar<trailing_ws><no_final_newline>', 2)
Contrast this with the following behavior:
 [m.span() for m in re.compile('(?P<trailing_ws>[ \t]+\r*$)|(?P<no_final_newline>(?<=[^\n])\Z)', re.M).finditer('foobar ')]
gives
 
 [(6, 7), (7, 7)]
The matches are clearly not overlapping and the re module docs for sub say "Return the string obtained by replacing the leftmost non-overlapping occurrences of pattern in string by the replacement repl.", so I would have expected two replacements.
This seems to be what perl is doing:
 echo -n 'foobar ' | perl -pe 's/(?m)(?P<trailing_ws>[ \t]+\r*$)|(?P<no_final_newline>(?<=[^\n])\Z)/<$&>/g' 
gives
 foobar< ><>%