For me one of the most common issues is when I write a regex but I have some of the escapes the wrong way around etc. So I’m giving a valid regex, but it’s not matching what I tried to match. And then I have to change it around a couple of times before it works.

One reason for this is that different programs have different rules for what should be escaped when, when you write a regex. For example, I think grep is a bit different from vim in this regard

Hm... Now that I think about it, regex tools on the command line haven't been a goto recently. I used sed, awk, perl -e, et. al. constantly from the late 90s until maybe 2015 but since then I'm more likely to pop open a ipython repl or whatever and avoid those weird inconsistencies altogether. Also, developing on the old scp->LAMP stack setup required more shell script glue than the more automated contemporary setups I've been using.

I'd probably suck at it. No matter how frequently I use regex in :ex commands, I always screw up the escapes, substitutions, etc.

It doesn't, I think it should but not sure how to add it to the prompt yet - I have a feeling that if I'll just add them after or before the command line, GPT will at least occasionally add hallucinated return codes to the autocompletion. Maybe I'll just add the unsuccessful codes or something, but it needs some testing.

Cool. Neat tool idea.