bug#33205: 26.1; unibyte/multibyte missing in rx.el

[Eli Zaretskii <eliz@gnu.org>]

> From: Mattias Engdegård <mattiase@acm.org>
> Cc: 33205@debbugs.gnu.org
> Date: Wed, 31 Oct 2018 16:27:53 +0100
> 
> tis 2018-10-30 klockan 19:27 +0200 skrev Eli Zaretskii:
> > I think it's a documentation bug: [:unibyte:] matches only ASCII
> > characters.  IOW, it tests "unibyteness" in the internal
> > representation (which might be surprising, I know).
> > 
> > And [:nonascii:] is only defined for multibyte characters.
> 
> Thus [:ascii:]/[:nonascii:] cannot be distinguished from
> [:unibyte:]/[:multibyte:]. Surely this cannot have been the intention?

I actually looked into this some more, and I think my original
conclusion was wrong.  Let me dwell on that a bit more, and I will
report what I found.  We can then revisit the questions you ask above.

> Taking a step back: Do you agree that the missing unibyte/multibyte
> should be added to rx

I think it depends on what we find regarding the functionality.  It's
possible that it makes no real sense in the context of rx, for example
(although it indeed sounds like an omission).

> If there is a useful interpretation of [:unibyte:]/[:multibyte:] today,
> perhaps we could make them behave that way. 

Right.  Stay tuned, and thanks for pointing out this surprising
behavior.