std::regex r(R"((b[[:alpha:]]+b)|(bd+b))");
std::string s = "foo bar 123";
for(std::sregex_iterator i = std::sregex_iterator(s.begin(), s.end(), r);
                         i != std::sregex_iterator();
                         ++i)
{
    std::smatch m = *i;
    std::cout << "Match value: " << m.str() << " at Position " << m.position() << 'n';

    for(auto index = 1; index < m.size(); ++index ){
        if (!m[index].str().empty()) {
            std::cout << "Capture group ID: " << index-1 << std::endl;
        }
    }
}

Match value: foo at Position 0
Capture group ID: 0
Match value: bar at Position 4
Capture group ID: 0
Match value: 123 at Position 8
Capture group ID: 1