Cocoa Touch Tutorial: Stripping Non-Alphanumeric Characters on Entry in a UITextField

In a previous post, I showed you how to trim non-alphanumeric characters from a string. Here I’ll go more in-depth and show a method that I wrote to restrict text entry in a UITextField to alphanumeric characters. Since I also wanted the characters to be uppercase, I’ll also ensure that only uppercase characters are allowed.

This should all happen in the - (BOOL)textField:(UITextField *)textField shouldChangeCharactersInRange:(NSRange)range replacementString:(NSString *)string method of your UITextField’s delegate (which, of course, must implement the UITextFieldDelegate protocol). I’ve implemented it as follows:

- ( BOOL )textField:( UITextField * )textField
shouldChangeCharactersInRange:( NSRange )range
  replacementString:( NSString * )string
{
    /*
     * We only want uppercase letters and numbers in this text field, so if
     * this method is adding something else, we don't want it. But we also
     * want to support copy-and-paste, so it's not always going to be one
     * character added.
     */
    BOOL shouldAllowChange = YES;

The shouldAllowChange variable is set to YES initially because we want to allow this change when possible. The method will test the string to see if it meets criteria for rejection as we move forward.

    NSMutableString *newReplacement =
    [[ NSMutableString alloc ] initWithString:[ string uppercaseString ]];

    if ( ! [ string isEqualToString:newReplacement ]) {
        shouldAllowChange = NO;
    }

First, we define newReplacement. It’s an NSMutableString so that if we discover non-alphanumeric characters in it, we can remove them on-the-fly. It also serves as a convenient string against which we can test to see if string is already uppercase.

    NSCharacterSet *desiredCharacters =
    [ NSCharacterSet alphanumericCharacterSet ];

    for ( NSUInteger i = 0; i < [ newReplacement length ]; i++ ) {
        unichar currentCharacter = [ newReplacement characterAtIndex:i ];

        if ( ! [ desiredCharacters characterIsMember:currentCharacter ]) {
            shouldAllowChange = NO;
            [ newReplacement deleteCharactersInRange:NSMakeRange( i, 1 )];
            i--;
        }
    }

In this section, we define the NSCharacterSet that we want to work with - in this case, the alphanumeric character set. We go through one character by a time and if the current character isn’t alphanumeric, we remove it from the NSMutableString (decrementing i so that we don’t inadvertently skip a character) and set our shouldAllowChange flag accordingly.

    if ( shouldAllowChange ) {
        [ newReplacement release ];
        return YES;
    } else {
        [ textField setText:[[ textField text ]
                             stringByReplacingCharactersInRange:range
                             withString:newReplacement ]];
        [ newReplacement release ];
        return NO;
    }
}

To finish, if shouldAllowChange is still true, we return YES and allow the replacement characters to be added. Otherwise, we return NO, but not before using our replacement replacement string (say that ten times fast) to manually edit the text field’s text. The end result is a text field that will consist only of uppercase letters and numbers.