How can I use the highlight-chars.el
package mentioned here (and here's its code) to highlight custom characters?
Specifically, I'd like to highlight the unicode directional formatting characters for embedding right-to-left languages in a predominantly left-to-right text and vice versa.
Before you say "But what do you need the directional formatting characters for? The newer versions of Emacs handle bidirectional text natively.", I'd like to remind you that there is a reason why the unicode algorithm allows for explicit bidirectional markers.
hc-highlight-chars
. Did you try this?