[Python-Dev] Zero-width matching in regexes

[Serhiy Storchaka]

05.12.17 01:21, MRAB пише:
> I've finally come to a conclusion as to what the "correct" behaviour of 
> zero-width matches should be: """always return the first match, but 
> never a zero-width match that is joined to a previous zero-width match""".
> 
> If it's about to return a zero-width match that's joined to a previous 
> zero-width match, then backtrack and keep on looking for a match.

Isn't this how sub(), findall() and finditer() work in regex with 
VERSION1? I agree that this behavior looks most logical and self-consistent.

Unfortunately the different behavior of re.sub() is documented explicitly:

"Empty matches for the pattern are replaced only when not adjacent to a 
previous match, so sub('x*', '-', 'abc') returns '-a-b-c-'."

And there a special purposed test for this. One time the behavior was 
changed when the re implementation was changed from pre to sre, but the 
older behavior was restored. [1] [2]

[1] https://bugs.python.org/issue462270
[2] 
https://github.com/python/cpython/commit/21009b9c6fc40b25fcb30ee60d6108f235733e40