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.

Thanks for posting about Regexle! My buddy and I made it last year, inspired by Jimbly, but we regularly forget it's still up. Stoked that folks are enjoying it. If you have any suggestions for improvements, we're open to em! https://gitlab.com/Nathaniel.Belles/regexle/-/issues

[^X]* means anything but X as many times as you want. So F will match this part.

[^X] means any character other than X, so [^X]* matches zero or more of any characters, so long as none of them are X.

BIG EDIT:

Because it's so trivially unsolvable, I had a quick look at the posted solution.

We're reading it wrong.

Take the first regex on the top row, one you included in your example:

[^X]*(DN|TE|NI)

You're supposed to fill that in from the bottom to the top.

In other words, the first letter goes immediately to the right of [^c]*[^R]*III.* - that square has to be a 'not X'. The hex immediately below it has to be an E, I, or N.

Yeah, that's how I found regexle, which eventually led me to this!

The worst is when you're an hour in and there's some crucial wrong assumption you've made at some point that's cascaded across the board and you have to start again. Had to come back to it a day later, but very satisfying to complete. I must be a masochist because I want to do another.