C++::Boost::Regex Iterate over the submatches

Question

I am using Named Capture Groups with Boost Regex / Xpressive.

I would like to iterate over all submatches, and get both the value and KEY of each submatch (i.e. what["type"]).

sregex pattern = sregex::compile(  "(?P<type>href|src)=\"(?P<url>[^\"]+)\""    );

sregex_iterator cur( web_buffer.begin(), web_buffer.end(), pattern );
sregex_iterator end;

for( ; cur != end; ++cur ){
    smatch const &what = *cur;

    //I know how to access using a string key: what["type"]
    std::cout << what[0] << " [" << what["type"] << "] [" << what["url"] <<"]"<< std::endl;

    /*I know how to iterate, using an integer key, but I would
      like to also get the original KEY into a variable, i.e.
      in case of what[1], get both the value AND "type"
    */
    for(i=0; i<what.size(); i++){
        std::cout << "{} = [" << what[i] << "]" << std::endl;
    }

    std::cout << std::endl;
}