you like regex or want to get better?

there you go RegexCrossword
if you have difficulties to read the vertical regex hover it...

* if you press Help on top you get a nice helplist with common pattern matcher

*edit:
if your brain goes totally out of control you can verify your ideas with that nice live matcher:
https://regex101.com/ (make sure to select javascript regex)

maybe you did that:
http://regex.alf.nu/
or that:
http://regexone.com/
or that:
http://jimbly.github.io/regex-crossword/
regexone is pretty good with learning first things...

This should be valid, right? All four conditions are met.
If yes: website buggy/silly, 'cause it wont let me pass.
If no: where is my mistake?

quote: "no character" is not allowed.

Which of the regex rules says so? Where is the conflict?

quote: where you must guess the correct letters

/ and * are not letters either though. Neither are numbers.

quote: Can you just take it Skasi that there has to be at least one character...?

There should not be any hidden conditions, only the ones visible on the screen.

(psst, I am just sad/disappointed that the game does not reward (my) creativity)

Nah, the game is. As are responses that neither confirm that my input is valid nor explain where I made a mistake. (I'm not a regex pro)

I'm fine with entering O to continue to the next puzzle, the game is not fine with me not entering O. It's a puzzle about logic and being technically correct, yet it isn't. Mind you, I am not saying you should all stop visiting the website and stop having fun solving the puzzles, I am just pointing out a little "quirk" I noticed/discovered.

Yay!

The bottom vertical one? Has to be

. SAI..

not

. SAT..

click

I think he confused [] with () ... just a concentration thing. :)
I kinda solved it different, but do not remember how... I didn't need to think about I or T there.