RE: empty-directory predicate, native implementation

[Drew Adams <drew.adams@oracle.com>]

> > `directory-files-no-dot-files-regexp' was added to Emacs 23.
> > Its value then was the same as that of `diredp-re-no-dot':
> > "^\\([^.]\\|\\.\\([^.]\\|\\..\\)\\).*".  The value was
> > changed in Emacs 27, to "[^.]\\|\\.\\.\\.".
> >
> > For my purposes (Dired) I want the former, not the latter,
> > so `diredp-re-no-dot' remains the former.  The two behave
> > quite differently.
> >
> > See https://urldefense.com/v3/__https://lists.gnu.org/archive/html/emacs-
> devel/2020-
> 04/msg00764.html__;!!GqivPVa7Brio!PWSpyl3EDfFC2LKEXIP7mdNKcGl6HzDDLwE2SFOWdxS
> ZaQ3phv5AVvVuUb-CN6kG$
> 
> FTR, I have also problems to understand how the current value of
> directory-files-no-dot-files-regexp works. But I fail to find a case
> where it is wrong.
> 
> (string-match directory-files-no-dot-files-regexp ".") => nil
> (string-match directory-files-no-dot-files-regexp "..") => nil
> (string-match directory-files-no-dot-files-regexp ".a") => 1
> (string-match directory-files-no-dot-files-regexp "..a") => 2
> 
> Could you pls give me an example which shows the problem with that
> constant? In case there is, I'll lobby for your request in the given
> message :-)

Dunno.  And perhaps I misspoke in saying they behave quite
differently.  They _can_ behave quite differently, depending
on how they're used.

And frankly I think that the only current Dired+ uses of the
regexp don't depend on the difference, as they all just pass
it to `directory-files' as the MATCH arg.  And in that case
the new regexp is just as usable.

In general, the difference between the two is this, AFAICT:
the old one (which is the one still used by Dired+) matches
the complete file name (the nondirectory part), whereas the
new one matches only enough of it to distinguish the . and
.. cases.

IOW, what's different, AFAICS, is the match data: the match.

So if you use the regexp only with `string-match-p' (which
doesn't care about the match data), or if you use it only
with `directory-files', then there's no real difference in
the effect.  But if you use it for some context where the
matched parts are important, that is, where the match-data
matters, then there's a big difference.

Perhaps in the past I used the regexp also for purposes of
grabbing the matched part(s).  I don't recall.

I didn't complain about Emacs changing the value of the
variable - no lobbying is needed.  What I said was that
"it's not clear to me" why people were claiming that the
new regexp is "more correct" than the old one.  (No one
ever responded to that, explaining in what way the old
one was somehow incorrect.)

Paul's mail responding to my mail in that emacs-devel
thread says, BTW:

  As Drew's comments make evident, the doc string is
  unclear. It should be something like 'Regexp that
  matches part of a nonempty string if the string is
  ^^^^^^^^^^^^
  neither "." nor "..".'

But I couldn't find where in that thread I said that.

Anyway, I've said it now.  The old regexp matches all
chars in the nondir part of the file name.  The new
regexp doesn't.  The match data for the old regexp
gives you the matched name.  But no, that's not
needed for `directory-file-names'.
___

[BTW, neither manual nor doc string for `directory-files'
says what MATCH is matched against, other than "file names".
But apparently it's matched only against the nondirectory
part of file names, even if FULL is non-nil.]