(?=foo) Lookahead Asserts that what immediately follows the current position in the string is foo (?<=foo) Lookbehind Asserts that what immediately precedes the current position in the string is foo (?!foo) Negative Lookahead Asserts that what immediately follows the current position in the string is not foo (?<!foo) Negative Lookbehind Asserts that what immediately precedes the current position in the string is not foo