UITextField format in xx-xx-xxx

Had a need to do this nicely for a phone numbers with a variable format, here’s what I wrote. Feel free to reuse – First I’ve got a method to filter a formatted string, with # being a number, and any other character being some kind of “filler” that should be conveniently inserted after the user gets to the place where it’s needed.

NSMutableString *filteredPhoneStringFromStringWithFilter(NSString *string, NSString *filter)
{
    NSUInteger onOriginal = 0, onFilter = 0, onOutput = 0;
    char outputString[([filter length])];
    BOOL done = NO;

    while(onFilter < [filter length] && !done)
    {
        char filterChar = [filter characterAtIndex:onFilter];
        char originalChar = onOriginal >= string.length ? '\0' : [string characterAtIndex:onOriginal];
        switch (filterChar) {
            case '#':
                if(originalChar=='\0')
                {
                    // We have no more input numbers for the filter.  We're done.
                    done = YES;
                    break;
                }
                if(isdigit(originalChar))
                {
                    outputString[onOutput] = originalChar;
                    onOriginal++;
                    onFilter++;
                    onOutput++;
                }
                else
                {
                    onOriginal++;
                }
                break;
            default:
                // Any other character will automatically be inserted for the user as they type (spaces, - etc..) or deleted as they delete if there are more numbers to come.
                outputString[onOutput] = filterChar;
                onOutput++;
                onFilter++;
                if(originalChar == filterChar)
                    onOriginal++;
                break;
        }
    }
    outputString[onOutput] = '\0'; // Cap the output string
    return [NSString stringWithUTF8String:outputString];
}

Now so that they can delete through filler, I modified my should change characters in range.

- (BOOL)textField:(UITextField *)textField shouldChangeCharactersInRange:(NSRange)range replacementString:(NSString *)string {

    NSString *filter = @"(###) ### - ####";

    if(!filter) return YES; // No filter provided, allow anything

    NSString *changedString = [textField.text stringByReplacingCharactersInRange:range withString:string];

    if(range.length == 1 && // Only do for single deletes
       string.length < range.length &&
       [[textField.text substringWithRange:range] rangeOfCharacterFromSet:[NSCharacterSet characterSetWithCharactersInString:@"0123456789"]].location == NSNotFound)
    {
        // Something was deleted.  Delete past the previous number
        NSInteger location = changedString.length-1;
        if(location > 0)
        {
            for(; location > 0; location--)
            {
                if(isdigit([changedString characterAtIndex:location]))
                {
                    break;
                }
            }
            changedString = [changedString substringToIndex:location];
        }
    }

    textField.text = filteredPhoneStringFromStringWithFilter(changedString, filter);

    return NO;
}

This provides a really clean way to force users to enter numbers in a particular format.