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Tue Dec 5 15:26:04 EST 2017 · https://mail.python.org/pipermail/python-dev/2017-December/151067.html

On 12/4/2017 6:21 PM, MRAB wrote:
> 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""".

Is this different from current re or regex?

> 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.
> 
> Example:
> 
>  >>> print([m.span() for m in re.finditer(r'|.', 'a')])
> [(0, 0), (0, 1), (1, 1)]
> 
> re.findall, re.split and re.sub should work accordingly.
> 
> If re.finditer finds n matches, then re.split should return a list of 
> n+1 strings and re.sub should make n replacements (excepting maxsplit, 
> etc.).

-- 
Terry Jan Reedy

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[Python-Dev] Zero-width matching in regexes