bug#43702: Emacs master: Incorrect highlighting in regexp isearch.

[Alan Mackenzie]

Hello, Emacs.

In the Emacs master branch
(i) emacs -Q
(ii) C-x C-f emacs/src/syntax.c<CR>
(iii) C-M-s \(inc\|dec\)_both
(iv) Press C-s a few times.

Notice that the highlighting of the found match has the purple
background over the correct characters, but the characters displayed in
the foreground are "   _both".  This incorrect display is a bug.

(v) Press <backspace> a few times.  The lazy highlighting is now
correctly displayed each time.
(vi) Note that if one starts at the end of the buffer with C-M-r, etc.,
the same problem occurs.

-- 
Alan Mackenzie (Nuremberg, Germany).