[Python-Dev] Zero-width matching in regexes (original) (raw)

Serhiy Storchaka storchaka at gmail.com
Wed Dec 6 06:58:27 EST 2017

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

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