bug#34911: 26.1; doc about lock file names

[Drew Adams]

> >> The default value is "^\\.?#\\|^\\.$\\|^\\.\\.$".
> >>
> >> \\.?# matches only the first char of an auto-save file
> >> name, and the first two chars of a lock file name.  It
> >> does not match the full name, requiring it to end with
> >> `#'.  That means that (1) it cannot be used as is for,
> >> say, font-locking such a (complete) name, and (2) as
> >> it is now, it can falsely identify files that are not
> >> auto-save or lock files.
> >
> > It's a regexp, so of course you can name files which aren't auto-save or
> > lock files in such a way that they match it.
> 
> After looking at it some more, I realize you were probably suggesting
> that the regexp should have something like "^.#\\|^#.*#$" instead of
> "^\\.?#"

No, I don't think so.  It is correct, I think, to
say that the beginning matches .# (or the whole
thing matches . or ..) - that is, the dot is literal.

What I was saying was that it is not enough to say
how it starts.  It should (in all cases) match the
full file name.  (And it is better to use \` and \'.)

A main reason is what I mentioned: being able to
reuse the variable for font-locking.

I do that in `dired+.el', for example.  Option
`diredp-omit-files-regexp' is a regexp to font-lock
file names omitted by `dired-omit-mode'.

The default value is defined this way, reusing the
value of `dired-omit-files':

(let* ((strg  dired-omit-files) ; <=========== Reuse it
       (strg  (if (eq ?^ (aref strg 0)) ; Remove initial ^
                  (substring strg 1)
                strg))
       (strg  (replace-regexp-in-string
               "\\(\\\\[|]\\)\\^"       ; Remove other ^'s
               "\\1"
               strg
               'FIXEDCASE
               nil))
       (strg  (replace-regexp-in-string
               "\\([$]\\)"              ; Remove $'s
               ""
               strg
               'FIXEDCASE
               nil)))
  strg)

Doc string:

Regexp for font-locking file names to be omitted by `dired-omit-mode'.
The regexp is matched only against the file name, but the entire line
is highlighted (with face `diredp-omit-file-name').

The default value of this option differs from that of
`dired-omit-files' by removing "^" from the beginning, and "$"
from the end, of each regexp choice.  (The default value of
`dired-omit-files', at least prior to Emacs 27, uses "^" and "$",
but it should not.)

If you want to control the beginning and end of choice matches then
use "\`" and "\'" instead of "^" and "$".

Note: If you change the value of this option then you need to restart
Emacs to see the effect of the new value on font-locking.