bug#31492: 26.1; query-replace-regexp undo fails in regexps w/o printable chars

[Tino Calancha]

On Fri, 18 May 2018, Eli Zaretskii wrote:

> > From: Tino Calancha <tino.calancha@gmail.com>
> > Date: Fri, 18 May 2018 23:22:13 +0900 (JST)
> > cc: Tino Calancha <tino.calancha@gmail.com>, 31492@debbugs.gnu.org
> >
> > > > emacs -Q
> > > >
> > > > < C-M-% \b RET foo RET TAB TAB TAB
> > > > U ; undo all replacements
> > > > ;; Nothing is undid :-(
> > '<' stands for `beginning-of-buffer'.
> Sorry, I'm still in the dark. Typing '<' inserts that character
Opps... Sorry!  That should read 'M-<'.
And the TAB's should read SPC.

The correct recipe is:

M-<
C-M-% \b RET foo RET SPC SPC
U
;; All 'foo' keep there :-(

This happen because the regexp "\b" has any printable character.
If we try instead:

M-<
C-M-% is\b RET foo RET SPC SPC
U
;; Now all 'foo' are gone :-)