iOS automatically add hyphen in text field

Be aware the previous answer is woefully inadequate. Heaven forbid your user enter an incorrect digit and dare attempt to delete it! In fairness, the poster noted the code may not work perfectly. But then, it wouldn’t even compile, so the buyer beware filter should already be high. If you fix the compile error and try the code, you’ll see you can easily end up with input that does not match the poster’s stated format.

Here’s a solution I’ve used for restricting a text field to a phone number of the format 123-456-7890. Adjusting for other numeric formats is trivial. Note the use of the passed NSRange. And BTW, rejecting non-digit characters is needed even when using a numeric virtual keyboard since users can still enter non-digits via a hardware keyboard.

One other note. I add the hyphen after the entry of the 4th and 7th digits to make the deleting of digits a bit easier. If you add after the 3rd and 6th digits, you will have to handle the case of deleting the dangling hyphen. The code below avoids that use case.

// Restrict entry to format 123-456-7890
- (BOOL)                textField:(UITextField *)textField
    shouldChangeCharactersInRange:(NSRange)range
                replacementString:(NSString *)string {

  // All digits entered
  if (range.location == 12) {
    return NO;
  }

  // Reject appending non-digit characters
  if (range.length == 0 &&
       ![[NSCharacterSet decimalDigitCharacterSet] characterIsMember:[string characterAtIndex:0]]) {
    return NO;
  }

  // Auto-add hyphen before appending 4rd or 7th digit
  if (range.length == 0 &&
      (range.location == 3 || range.location == 7)) {
    textField.text = [NSString stringWithFormat:@"%@-%@", textField.text, string];
    return NO;
  }

  // Delete hyphen when deleting its trailing digit 
  if (range.length == 1 &&
      (range.location == 4 || range.location == 8))  {
    range.location--;
    range.length = 2;
    textField.text = [textField.text stringByReplacingCharactersInRange:range withString:@""];
    return NO;
  }

  return YES;
}