Re: Unicode confusables and reordering characters considered harmful, a simple solution

[Eli Zaretskii <eliz@gnu.org>]

> Date: Wed, 03 Nov 2021 21:16:56 +0000
> From: Gregory Heytings <gregory@heytings.org>
> cc: cpitclaudel@gmail.com, yuri.v.khan@gmail.com, stefan@marxist.se, 
>     emacs-devel@gnu.org, db48x@db48x.net, monnier@iro.umontreal.ca, 
>     juri@linkov.net
> 
> >> I tried various of the predefined colors with highlight-regexp, of 
> >> course those with a distinct background, and none make any of those 
> >> characters more visible.  Not even with a single pixel wide bar.
> >
> > That's very strange, because I see them even if I make the region span 
> > only a single such character.  The default background of the region face 
> > is quite pale, and still I see them quite clearly, even with the default 
> > light theme.
> >
> 
> Here's a screenshot with emacs -Q (current trunk).  I did M-x 
> highlight-regexp RET y RET hi-green RET M-x highlight-regexp <regexp with 
> all reordering characters> RET hi-green RET.  There's one reordering 
> character between each of the "abcdefghij" string.  I zoomed that picture, 
> I see absolutely nothing.

If you configured highlight-regexp to highlight only the formatting
controls, how come 'y' is highlighted in green on the image you sent?
Is 'y' one of the characters that are supposed to be highlighted?

Or maybe your configuration of highlight-regexp was incorrect?  Or
could there be some subtle bug/misfeature in highlight-regexp (I
didn't try it myself)?

If you shift-highlight one of these formatting control characters, and
only one such character, don't you see a thin whitespace shown in the
background color of the region face?