[regex] Regular Expressions and negating a whole character group

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 [^ab], [^(ab)], 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?

Using a character class such as [^ab] will match a single character that is not within the set of characters. (With the ^ being the negating part).

To match a string which does not contain the multi-character sequence ab, you want to use a negative lookahead:

^(?:(?!ab).)+$

And the above expression disected in regex comment mode is:

(?x)    # enable regex comment mode
^       # match start of line/string
(?:     # begin non-capturing group
  (?!   # begin negative lookahead
    ab  # literal text sequence ab
  )     # end negative lookahead
  .     # any single character
)       # end non-capturing group
+       # repeat previous match one or more times
$       # match end of line/string