Why does std::regex_match not support "zero-length assertions"?

#include <regex>

int main()
{
    b = std::regex_match("building", std::regex("^\w*uild(?=ing$)"));
    //
    // b is expected to be true, but the actual value is false.
    //
}

My compiler is clang 3.8.

Why does std::regex_match not support "zero-length assertions"?