(?=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
F. Dau. Proceedings of the 10th International Conference on Conceptual Structures (ICCS 2002), volume 2393 of Lecture Notes in Computer Science, page 326-340. Springer, (2002)
S. Boytcheva, A. Strupchanska, and G. Angelova. Proceedings of the 10th International Conference on Conceptual Structures (ICCS 2002), volume 2393 of Lecture Notes in Computer Science, page 137-150. Springer, (2002)
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)
M. Leclère, and M. Mugnier. Proceedings of the 16th International Conference on Conceptual Structures (ICCS 2008), volume 5113 of Lecture Notes in Computer Science, page 119-132. Springer, (2008)