Brackets are character classes. They match any single character between them--specific meta characters are interpretted differently. They allow ranges such as a-z for matching the english alphabet (lowercase), for example. They also support negation by placing ^ as the first character, ie, match anything BUT the following. You might want to play around here: https://regexr.com/

But if height was a negative number and bigger than window, multiplying it by bounce would cause an infinite loop. I think.