I want to call out https://www.regular-expressions.info/ since it was, for a long time, one of the better/best resources on regexes that I was able to find. I learned a lot from this ... guy, essentially.
but, I always try to add a warning when recommending - should use them only for the flavors supported (PCRE, Python, etc) I've seen many using it for cli tools and wonder why things like non-greedy or lookarounds don't work.
It's such a good resource for understanding and writing both simple and complicated regex.