... unless the string ends with the target character/pattern

That's why I said it only worked in this special case, not ending in j. But I agree with you, if you're not trying to golf, (*FAIL) is probably the best option, because it is far more explicit even if you've never seen the construct before. (?!) is still weird enough that you still need to check in the documentation if you don't recognize it, so that's still good.