Validate
        Perform value validation and return result

        :param value:           value to check, cast to string
        :param model:           parent model being validated
        :param context:         object or None, validation context
        :return:                shiftschema.results.SimpleResult

def validate(self, value, model=None, context=None):
        """
        Validate
        Perform value validation and return result

        :param value:           value to check, cast to string
        :param model:           parent model being validated
        :param context:         object or None, validation context
        :return:                shiftschema.results.SimpleResult
        """
        import re

        value = str(value)
        match = re.match(r'^\d+', value)
        if not match or value != match.group():
            return Error(self.not_digital)

        # success otherwise
        return Error()