No, but variable size lookahead/behind do. This is because the engine has to go back if the remaining part of a regex fails. For some examples, see http://www.r

No, but variable size lookahead/behind do. This is because the engine has to go back if the remaining part of a regex fails. For some examples, see http://www.regular-expressions.info/recursebacktrack.html http://www.regular-expressions.info/recursebacktrack.html) EDIT: you are correct, backreferences do require backtracking, my bad.