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< ><>%

It's a bug caused by trying to avoid getting stuck when a zero-width match is found. Basically the fix is to advance one character after a zero-width match, but that doesn't always give the correct result.

There are a number of related issues like issue bpo-1647489 ("zero-length match confuses re.finditer()").

@serhiy can you take a look at this as I recall you've been doing some regex work?

This bug was fixed in Python 3.7, see bpo-32308.

Python 3.6.8 (tags/v3.6.8:3c6b436a57, Dec 23 2018, 23:31:17) [MSC v.1916 32 bit (Intel)] on win32
>>> 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 ')
('foobar<trailing_ws>', 1)

Python 3.7.3rc1 (tags/v3.7.3rc1:69785b2127, Mar 12 2019, 22:37:55) [MSC v.1916 64 bit (AMD64)] on win32
>>> 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 ')
('foobar<trailing_ws><no_final_newline>', 2)