Example
A positive lookahead (?=123) asserts the text is followed by the given pattern, without including the pattern in the match. Similarly, a positive lookbehind (?<=123) asserts the text is preceded by the given pattern. Replacing the = with ! negates the assertion.
Input: 123456
123(?=456)matches123(positive lookahead)(?<=123)456matches456(positive lookbehind)123(?!456)fails (negative lookahead)(?<!123)456fails (negative lookbehind)
Input: 456
123(?=456)fails(?<=123)456fails123(?!456)fails(?<!123)456matches456
