How do I negate an exact match in regex?

Question

If the user enters exactly "null", I want the regex match to fail. Entering "xxxnullxxx" is ok however.

The following regex rejects "null" but it also rejects any string containing "null", which I don't want.

^(?!.*null).*$

Answer 1

Add $ and remove .* from the look ahead:

^(?!null$).*

The trailing $ isn't needed.