Fix what looks like an off-by-one error in RegEx::longestMatch()'s lastMatch calculation, and a corresponding +1 in code using longestMatch, and add a test.

main.cpp

@@ -19,6 +19,7 @@
 int main(int argc, char* argv[]) {
 	if (argc == 2 && std::string(argv[1]) == "--test") {
 		StringReader::test();
+		RegEx::test();
 		Lexer::test();
 		return 0;
 	}