Re: Adding new escapes to regex module

On 2022-08-17 08:25, Serhiy Storchaka wrote:
> 16.08.22 23:24, MRAB пише:
>> Other regex implementations have escape sequences for horizontal 
>> whitespace (`\h` and `\H`) and vertical whitespace (`\v` and `\V`).
>> 
>> The regex module already supports `\h`, but I can't use `\v` because it 
>> represents `\0x0b', as it does in the re module.
>> 
>> Now that someone has asked for it, I'm trying to find a nice way of 
>> adding it, and I'm currently thinking that maybe I could use `\y` and 
>> `\Y` instead as they look a little like `\v` and `\V`, and, also, 
>> vertical whitespace is sort-of in the y-direction.
>> 
>> As far as I can tell, only ProgressSQL uses them, and, even then, it's 
>> for what everyone else writes as `\b` and `\B`.
>> 
>> I want the regex module to remain compatible with the re module, in case 
>> they get added there sometime in the future.
>> 
>> Opinions?
> 
> I do not like introducing escapes which are not supported in other RE
> implementations. There is a chance of future conflicts.
> 
> Java broke compatibility in Java 8 by redefining \v from a single
> vertical tab character to the vertical whitespace class. I am not sure
> that it is a good example that we should follow, because different
> semantic of \v in raw and non-raw strings is a potential source of bugs.
> But with special flag which controls the meaning of \v it may be more safe.
> 
> Horizontal whitespace can be matched by [
> \t\xA0\u1680\u180e\u2000-\u200a\u202f\u205f\u3000] in re or [\t\p{Zs}]
> in regex. Vertical whitespace can be matched by
> [\n\x0b\f\r\x85\u2028\u2029]. Note that there is a dedicated Unicode
> category for horizontal whitespaces (excluding the tab itself), but not
> for vertical whitespaces, it means that vertical whitespaces are less
> important.
> 
> In any case it is simple to introduce special Unicode categories and use
> \p{ht} and \p{vt} for horizontal and vertical whitespaces.
> It's not just Java. Perl supports all 4 of \h, \H, \v and \V. That might 
be why Java 8 changed.
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