Regex negation?

Question

I\'m playing Regex Golf (http://regex.alf.nu/) and I\'m doing the Abba hole. I have the following regex that matches the wrong side entirely (which is what I was trying to

Answer 1

You can make it much shorter (and get more points) by simply using . and removing unnecessary parens:

^(?!.*(.)(.)\2\1)

It just makes sure that there's no "abba" ("abba" here means 4 letters in that particular order we don't want to match) in any part of the string without having to match the whole word.