title: NSCharacterSet: How do I add "_" to alphanumericCharacterSet text restriction?
body: <p>Building an NSCharacter set to restrict a UITextField for entering user names. I want the user to be able to also enter an underscore (so [A-Za-z0-9_]) but alphanumericCharacterSet does not include it. Is there a way to specify a range like that in short form? I see <code>+ (id)characterSetWithRange:(NSRange)aRange</code>, but I'm not really understanding how that would work.</p>
  
  <p>I've got a simple UITextField sub-class that I pass the character set to. The restriction works fine and doesn't allow the user to enter anything but alpha numeric. Just need to add the "_" to those allowances.</p>
  
  <pre><code>NSCharacterSet *characterSet = [NSCharacterSet alphanumericCharacterSet];
  [textField setAllowed:characterSet];
  [textField setFrame:frame];
  </code></pre>