(?=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
D. Tishkovsky, and R. Schmidt. Proceedings of the 6th International Semantic Web Conference and 2nd Asian Semantic Web Conference (ISWC/ASWC2007), Busan, South Korea, volume 4825 of LNCS, page 435--448. Berlin, Heidelberg, Springer Verlag, (November 2007)