How to match a sequence of whitespaces with c++11 regex

std::string str = "ahw    \t\n";
std::regex re(R"((\s)*)");
std::smatch mr;
if (std::regex_search(str, mr, re))
{
    std::cout << "match found: " << mr.size() << "\n";
    for (size_t i = 0; i < mr.size(); ++i)
    {
        std::string strrep = mr.str(i);
        int len = mr.length(i);
        std::cout << "index: " << i << "len : " << len << " string: '" << strrep << "'\n";
    }
}
std::string newStr = std::regex_replace(str, re, "");
std::cout << "new string: '" << newStr << "'\n";

result:

What I expect: only 1 match, strrep should be ' \t\n', and len should be len(strrep) = 6. But both vc12 and gcc4.9.2 show the above result.

What's wrong with my understand? How could I match the whitespace sequence ' \t\n'?