(?=.?[A-Z]+)(?=.?[a-z])(?=.[!@#$%^&?()])
Checks that the string contains at least one uppercase letter, one lowercase letter and one special character
First, (?=.*?[A-Z]+)
What precedes it here is an empty (zero-width) position, so it matches every empty position that satisfies (?=.*?[A-Z]+)
(?=.?[A-Z]+)(?=.?[a-z])
Here, among the empty positions that already satisfy the first condition, it further matches those that satisfy (?=.*?[a-z])
(?=.?[A-Z]+)(?=.?[a-z])(?=.[!@#$%^&?()])
Likewise
^.(?=.{6,16})(?=.\d)(?=.[A-Z]{2,})(?=.[a-z]{2,})(?=.[!@#$%^&?()]).*$
Negative lookahead
(?!)
Positive lookbehind: right to left
(?<=)
Negative lookbehind
(?<!)
