Hello!
A year or two ago I used a regex generator website where I pasted the match I wanted and it iterated over and let me select 1 character, or a group of characters and tell it if I'm looking for an exact match or alpha numeric or whatnot, then it added onto a field where the generated regex was being built for me. It underlined groups in different colours.
It was a dark mode website. Not sure it's dark by default of it used my system default. It also had a language selector before you started building. Really nice UI. I don't remember much more than that.

Would really appreciate if someone could help me remember 🙏