std::smatch match;
std::string B = "B";
std::array<std::regex,2> regex = {
    std::regex("(A)"),
    std::regex("(B)")
};
for(auto it = regex.begin(); it != regex.end(); ++it)
    if(std::regex_match(B, match, *it) && match.size() > 0)
        std::cout << match.str(1) << std::endl;