(?!pattern)
               A zero-width negative lookahead assertion.  For
               example /foo(?!bar)/ matches any occurrence of
               "foo" that isn't followed by "bar".  Note however
               that lookahead and lookbehind are NOT the same
               thing.  You cannot use this for lookbehind.

               If you are looking for a "bar" that isn't preceded
               by a "foo", /(?!foo)bar/ will not do what you
               want.  That's because the (?!foo) is just saying
               that the next thing cannot be "foo"--and it's not,
               it's a "bar", so "foobar" will match.  You would
               have to do something like /(?!foo)...bar/ for
               that.   We say "like" because there's the case of
               your "bar" not having three characters before it.
               You could cover that this way:
               /(?:(?!foo)...|^.{0,2})bar/.  Sometimes it's still
               easier just to say:

                   if (/bar/ && $` !~ /foo$/)