title: Confusing on regular expression using in NSPredicate with ISBN numbers example
body: <p>Apple document gives an example when describing how to use regular expression in NSPredicate.</p>
  
  <pre><code>NSArray *isbnTestArray = @[@"123456789X", @"987654321x", @"1234567890", @"12345X", @"1234567890X"];
  NSPredicate *isbnPredicate = [NSPredicate predicateWithFormat:@"SELF MATCHES '\\\\d{10}|\\\\d{9}[Xx]'"];
  NSArray *isbnArray = [isbnTestArray filteredArrayUsingPredicate:isbnPredicate];
  </code></pre>
  
  <p>My question is why it use <code>\\\\d</code> but not <code>\\d</code> or <code>\d</code> ?</p>