+int
+rte_regexdev_rule_db_export(uint8_t dev_id, char *rule_db) {
+	struct rte_regexdev *dev;
+
+	RTE_REGEXDEV_VALID_DEV_ID_OR_ERR_RET(dev_id, -EINVAL);
+	dev = &rte_regex_devices[dev_id];
+	RTE_FUNC_PTR_OR_ERR_RET(*dev->dev_ops->dev_db_export,
+				-ENOTSUP);
+	if (rule_db == NULL) {
+		RTE_REGEXDEV_LOG(ERR, "Dev %d rules can't be NULL\n",
+				 dev_id);
+		return -EINVAL;
+	}
+	return (*dev->dev_ops->dev_db_export)(dev, rule_db); }

The test if (rule_db == NULL) is not according to the description of the API
It is needed to be NULL in order to retrieve rule_db length 

* @param[out] rule_db
 *   Block of memory to insert the rule database. Must be at least size in
 *   capacity. If set to NULL, function returns required capacity.