title: Unexpected end of regex when ascii character
body: <p>Minimal Verfiable Example</p>
  
  <pre><code>  #include&lt;regex&gt;
    int main(){
     std::regex re("\\u_nic400_ib_ext_m_ib_ar_fifo_wr_mux/mux_0_1_out [0]");
  
      }
  </code></pre>
  
  <p>Why is this giving me a regex_error? My debugger's error message is unexpected end of regex when ascii character, but I just trying to match the literal above and I don't see where the issue is. </p>