A negative lookahead says, at this position, the following regex can not match.
Let's take a simplified example:
a Match: (?!b) succeeds
ac Match: (?!b) succeeds
ab No match: (?!b(?!c)) fails
abe No match: (?!b(?!c)) fails
abc Match: (?!b(?!c)) succeeds
The last example is a double negation: it allows a b followed by c. The nested negative lookahead becomes a positive lookahead: the c should be present.
In each example, only the a is matched. The lookahead is only a condition, and does not add to the matched text.
也就是说"ab aa ac ad"