iOS How do I limit a Text Field length AND acceptable characters?

- (BOOL)textField:(UITextField *)textField 
    shouldChangeCharactersInRange:(NSRange)range 
    replacementString:(NSString *)string
{
    NSString *newString = [textField.text 
        stringByReplacingCharactersInRange:range withString:string];
    return !([newString length] > 3);
}

(NSRange)range replacementString:(NSString *)string
{
    NSCharacterSet *nonNumberSet = [[NSCharacterSet 
        characterSetWithCharactersInString:@"0123456789."] invertedSet];
    return ([string stringByTrimmingCharactersInSet:nonNumberSet].length > 0);
}