title: Check if String contains alphanumeric
body: <p>I want to check the string if it contains alphanumeric only.(Both letters and number only).
  I tried using <code>NSCharacterSet *strCharSet = [NSCharacterSet characterSetWithCharactersInString:@"abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ1234567890"];</code></p>
  
  <p>I also tried this <code>NSString *string = @"[^a-zA-Z0-9]";</code> = no luck</p>
  
  <p>but it only checks if the string does not contain any characters or it contains only this characters. I need to required the user to put both letters and numbers.</p>
  
  <p>-update
  I have a textfield for password. I need the user to input both letters and password. But I can't check if the textfield contains BOTH letters and numbers. By using the code above, it only checks if the textfield's text contains only alphanumeric.</p>