Geriatric millennial software engineers like myself grew up on https://regexr.com/ when it was still a Flash application. Unless you’re a masochist that lives and breathes regular expressions, it’s kind of hard to develop a good regex without live feedback, which sites like https://regexr.com/ provide.
This is a companion discussion topic for the original entry at https://blog.tjll.net/a-beginners-guide-to-extending-emacs