def _validate_index_string(index_string: str) -> bool:
    """
    Handle validation of index string from the argument. 
    a string should be in format 't{%d}' or 'r{%d}' or 'd{%d}'
    
    Parameters:
        index_string: the string we want to validate
    Returns:
        a boolean indicating whether the index string is valid
    """
    if len(index_string) < 2:
        print("Invalid index string length!")
        return False
    elif index_string[0] != 't' and index_string[0] != 'r' and index_string[0] != 'd':
        print("Invalid index string prefix!")
        return False
    elif not index_string[1:].isnumeric():
        print("Index need to have a number suffix!")
        return False
    else:
        return True