Yesterday, I discovered regexle [1] and today found out that this was the inspiration for it.
I think I've seen this puzzle before, but I really enjoyed myself gradually solving it while doing other things in the last two hours. I'm sharing this for anyone else who wants a way to spend their afternoon!
[1]: https://regexle.com
I like that one better. Smaller. More manageable. I don't want to spend 2 hours deciphering the big one.
Took 4.5 min to solve #519. Would have been quicker if I started with a better strategy but it was my first time.