https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64441
Bug ID: 64441 Summary: A match_results returns an incorrect sub_match if the sub_match::matched is false Product: gcc Version: 5.0 Status: UNCONFIRMED Severity: normal Priority: P3 Component: libstdc++ Assignee: unassigned at gcc dot gnu.org Reporter: kariya_mitsuru at hotmail dot com Created attachment 34362 --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=34362&action=edit g++ -v Please see the following sample. ====================================== sample code ====================================== #include <iostream> #include <regex> int main() { const char s[] = "abc"; const std::regex re("(\\d+)|(\\w+)"); std::cmatch m; std::regex_search(s, m, re); std::cout << std::boolalpha; for (size_t i = 0, n = m.size(); i < n; ++i) { auto&& sub = m[i]; std::cout << i << ":" << sub.matched << ", str = '" << sub.str() << "', " "range = [" << sub.first - s << ", " << sub.second - s << ")" << std::endl; } } ========================================================================================= ============================= output ============================= 0:true, str = 'abc', range = [0, 3) 1:false, str = '', range = [-140734305427376, -140734305427376) 2:true, str = 'abc', range = [0, 3) ================================================================== cf. http://melpon.org/wandbox/permlink/SBoMF5UKYYa38Y4N The C++11 standard 28.10[re.results]/p.4 says, "Otherwise matched is false, and members first and second point to the end of the sequence that was searched." So, I think that the output should be ============================= output ============================= 0:true, str = 'abc', range = [0, 3) 1:false, str = '', range = [3, 3) 2:true, str = 'abc', range = [0, 3) ==================================================================