regex iterate through groups

regex regular_expression = "(.*?)_(.*?)_(.*?)";
input_string = "3_4_5";
match_results<std::string::const_iterator> iterator;
regex_search(input_string, iterator, regular_expression);
string entire_string = m[0];
string group1 = m[1];
string group2 = m[2];
string group3 = m[3];