This answer may be the original intention of the questioner.

Excuse me, how can I see the fixed test content?
I can't pass the 7-8 10-11.

Like filter()

should return 0?

That is to say, are only the letters listed above?
I think too much!

Literally:

Match a non-# character [^#]

followed this whole pattern recursively (?R)* repeated any number of times

followed by 1 or more # characters #+

OR 1 or more # characters at the start of the string \A#+

FYI, regex101.com is a great tool for explaining regex.

Can you explain the regular expression and the execution efficiency?

i forgive you

Good!

I don't understand that add_letters() is 'z'.

Sorry, I just found the f string in codewars recently, so I used the f string for this question。

Unfortunately, I can't choose "Best Practices" and "Clever at the same time.

['<','='][n==0]
That was impressive.