为什么消极的前视时间用And/或Pipe来表示

I have an And/Or regex i.e (PatternA|PatternB) in which I only take PatternA if PatternB does not exist (PatternB always comes after PatternA but is more important) so I put a negative lookahead in the PatternA Pipe.I have an And/Or regex i.e (PatternA|PatternB)