title: "incomplete universal character name" with stringWithUTF8String
body: <p>when i try to convert form utf-8 string to NSString like so:</p>
  
  <pre><code>NSString *s = [NSString stringWithUTF8String:"\U0627\U0644\U0641\U0631\U0646"];
  NSLog(@"%@", s);
  </code></pre>
  
  <p>i get the compile error:</p>
  
  <pre><code>incomplete universal character name
  </code></pre>
  
  <p>note that it sometime just works fine:</p>
  
  <pre><code>NSString *UAE = [NSString stringWithUTF8String:"\U0627\U0644\U0641\U0631\U0646"];
      NSLog(@"%@", UAE);
  </code></pre>
  
  <p>and the output:</p>
  
  <pre><code>الامارات
  </code></pre>
  
  <p>so why is that happening? please help.</p>