Allow characters and single-char strings in rx charsets

The `not' and `intersection' forms, and `or' inside these forms,
now accept characters and single-character strings as arguments.
Previously, they had to be wrapped in `any' forms.
This does not add expressive power but is a convenience and is easily
understood.

* doc/lispref/searching.texi (Rx Constructs): Amend the documentation.
* etc/NEWS: Announce the change.
* lisp/emacs-lisp/rx.el (rx--charset-p, rx--translate-not)
(rx--charset-intervals, rx): Accept characters and 1-char strings in
more places.
* test/lisp/emacs-lisp/rx-tests.el (rx-not, rx-charset-or)
(rx-def-in-charset-or, rx-intersection): Test the change.

1 files changed, 3 insertions, 0 deletions

diff --git a/etc/NEWS b/etc/NEWS
index 1e0422c761f..a7f3c3d2fe8 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -2124,6 +2124,9 @@ Both match any single character; 'anychar' is more descriptive.
 With 'or' and 'not', it can be used to compose character-matching
 expressions from simpler parts.
 
++++
+*** 'not' argument can now be a character or single-char string.
+
 ** Frames
 
 +++