std::regex rx{"^ab$", std::regex_constants::multiline};
    std::string val = "abc\nab\ncb\nab\n";
    auto it = std::sregex_iterator{val.begin(), val.end(), rx};
    auto end = std::sregex_iterator{};
    for (; it != end; ++it) {
        std::smatch match = *it;
        std::cout << match.str() << "\n";
    }