As someone who enjoys shell scripting I don't think I have ever used the "expr" command. this astonishes and baffles me because I used to read the man pages for fun, I even had a little script to pick a random page. So thanks a bunch for bringing it to my attention.
My only guess as to why I am unfamiliar with the command is that perhaps younger me failed to figure out "expression" means regular expression. It only clicked this time because of your comment and the see also: re_format link at the bottom.
Yeah it's slightly obscure for a lot of people I think: most who do know of it seem to associate it purely with arithmetic, and rightly use `$((...))` instead.
1: https://pubs.opengroup.org/onlinepubs/9699919799/utilities/e...