So short. It looks like Code Complete

Your codes are very clean

I like this one a lot !

like it!!!

I see you also couldn't be bothered to write out all the names lmao

Not with rec((x + 1) % 9, y + 1), but with rec((x + 1) % 9, y). But even in this case, to move to the next row, the recursion would need to walk through the current row once again. And only in the second walk, when it is on the last cell, it checks that the cell is filled, it moves to the next row. If (y + 1) is used, then the recursion would change the row every time it meets empty cell.

can you explain this task to me, please. Why or returns the amount?

Please use spoiler flag next time.

You can't and there aren't any here. The colon is a chracter in the string to be matched, the question mark renders the preceding element optional.

lol damn never knew you could do ternany conditions inside a regex pattern