How to match when letters aren't the same?

Question

Suppose I want to match four-letter words whose first and last letters are not the same. What is a regex that will do this?

For example, I tried this: ^(.)..[^\1]$ but got no results.

Answer 1

Negative lookahead solves your problem.

/^(.)..(?!\1).$/

https://regex101.com/r/MvqXdE/1