@Catija Basically, stuff like "no filtering" is a bad idea in challenges, because it entirely calls into question "what counts as filtering?". Can I filter the alphabet so that it only contains certain letters? Or, is it only "filtering" if I use a builtin that says it filters, such as Python's `filter` or Jelly's `Ƈ` and `f` commands?
Besides, a common reason for banning things is to try to make a challenge more interesting. Your challenge is already interesting, as the approaches you can take are incredibly varied. Jelly will likely brute-force filter it, but something like Python would b…