'Validation of a value
@param value: the value
@return: tuple (value, error)'

def __call__(self, value):

    if value:
        name = str(value).lower().strip()
        from s3fields import s3_all_meta_field_names
        if ((name != 'id') and (name not in s3_all_meta_field_names()) and self.PATTERN.match(name)):
            return (name, None)
    return (value, self.error_message)
