title: Regular Expressions and negating a whole character group
body: <p>I'm attempting something which I feel should be fairly obvious to me but it's not.  I'm trying to match a string which does NOT contain a specific sequence of characters.  I've tried using <code>[^ab]</code>, <code>[^(ab)]</code>, etc. to match strings containing no 'a's or 'b's, or only 'a's or only 'b's or 'ba' but not match on 'ab'.  The examples I gave won't match 'ab' it's true but they also won't match 'a' alone and I need them to.  Is there some simple way to do this?</p>