Regex for string not ending with given suffix

I have not been able to find a proper regex to match any string not ending with some condition. For example, I don't want to match anything ending with an a.

This matches

b
ab
1

This doesn't match

a
ba

I know the regex should be ending with $ to mark the end, though I don't know what should preceed it.

Edit: The original question doesn't seem to be a legit example for my case. So: how to handle more than one character? Say anything not ending with ab?

I've been able to fix this, using this thread:

.*(?:(?!ab).).$

Though the downside with this is, it doesn't match a string of one character.