@classmethod
def is_expression_attribute_name(cls, input_string):
    """
        https://docs.aws.amazon.com/amazondynamodb/latest/developerguide/Expressions.ExpressionAttributeNames.html
        An expression attribute name must begin with a pound sign (#), and be followed by one or more alphanumeric
         characters.
        """
    return input_string.startswith('#') and cls.is_expression_attribute(
        input_string[1:])
