title: NSRegularExpression giving warning
body: <p>Here is the code</p>
  
  <pre><code>NSError *error = nil;
  NSRegularExpression *regex = [NSRegularExpression regularExpressionWithPattern:@"[a-z0-9!#$%&amp;'*+/=?^_`{|}~-]+(?:\.[a-z0-9!#$%&amp;'*+/=?^_`{|}~-]+)*@(?:[a-z0-9](?:[a-z0-9-]*[a-z0-9])?\.)+(?:[A-Z]{2}|com|org|net|edu|gov|mil|biz|info|mobi|name|aero|asia|jobs|museum)" options:NSRegularExpressionCaseInsensitive error:&amp;error];
  </code></pre>
  
  <p>The code itself works but gives this warning:</p>
  
  <pre><code>"Unknown escape sequence \." 
  </code></pre>
  
  <p>I also tried the options:</p>
  
  <pre><code>NSRegularExpressionCaseInsensitive|NSRegularExpressionAnchorsMatchLines
  </code></pre>
  
  <p>but still the error persists. Can anyone explain why this error comes up and how it can be removed.</p>