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< ><>%