[BUG][GO] Generated validation uses deprecated validator.v2 causing regexp validation failures

[IceTweak]

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator?
• Have you tested with the latest master to confirm the issue still exists?
• Have you searched for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Sponsorship to speed up the bug fix or feature request (example)

Description

The Go code generator currently relies on the outdated gopkg.in/validator.v2 library, which lacks robust validation capabilities—especially for complex regex patterns. As a result, structs generated from OpenAPI specifications with regex-based validation rules often fail to validate correctly.

openapi-generator version

$ npx @openapitools/openapi-generator-cli version
7.14.0

OpenAPI declaration file content or url

Original repo

type: string
pattern: ^(?=.*\S.*)[^\x00-\x08\x0A-\x1f\x7f]{1,255}$
minLength: 1
maxLength: 255

Generation Details

Config:

generatorName: go
additionalProperties:
  packageName: notification
  withGoMod: false
  enumClassPrefix: true
  structPrefix: true

Generated result:

 import (
	"encoding/json"
	"fmt"

	"gopkg.in/validator.v2" // Generated package contains deprecated lib
)

...

type NotificationOrderItemDTO struct {
	OfferId string `json:"offerId" validate:"regexp=^(?=.*\\\\S.*)[^\\\\x00-\\\\x08\\\\x0A-\\\\x1f\\\\x7f]{1,255}$"`
	Count int32 `json:"count"`
}

And in UnmarshalJSON we have:

// try to unmarshal data into OrderCancelledNotificationDTO
	err = newStrictDecoder(data).Decode(&dst.OrderCancelledNotificationDTO)
	if err == nil {
		jsonOrderCancelledNotificationDTO, _ := json.Marshal(dst.OrderCancelledNotificationDTO)
		if string(jsonOrderCancelledNotificationDTO) == "{}" { // empty struct
			dst.OrderCancelledNotificationDTO = nil
		} else {
// This fails with: Items[0].OfferId: unknown tag
			if err = validator.Validate(dst.OrderCancelledNotificationDTO); err != nil {
// Here we lost error of unknown tag usage and cannot unmarshal right JSON to dedicated struct type
				dst.OrderCancelledNotificationDTO = nil
			} else {
				match++
			}
		}
	} else {
		dst.OrderCancelledNotificationDTO = nil
	}

Steps to reproduce

1. git clone https://github.com/yandex-market/yandex-market-notification-api.git
2. Use yaml config from Generation Details section above and run:

npx @openapitools/openapi-generator-cli generate \
  -c config.yaml \
  -i yandex-market-notification-api/openapi/openapi.yaml \
  -o client/

3. Go to a client/model_send_notification_request.go file and you will see deprecated library
4. Go to a client/model_notification_order_item_dto.go file and you will see usage of unsupported regexp tag
5. Try to unmarshal some vaild JSON, e.g.

{
    "campaignId": 123456,
    "orderId": 123456,
    "items": [
        {
            "count": 1,
            "offerId": "TEST-SKU-1"
        }
    ],
    "cancelledAt": "2025-07-29T09:02:46.452Z",
    "notificationType": "ORDER_CANCELLED"
}

And the UnmarshalJSON method will get into this branch (even though a completely valid JSON was provided):

else { // no match
		return fmt.Errorf("data failed to match schemas in oneOf(SendNotificationRequest)")
	}

Related issues/PRs

• [BUG] Golang one_of template is missing import for "gopkg.in/validator.v2"  #19089
• Fix Golang pattern validation with regex fails on commas #20079 #20369

Suggest a fix

If possible, why not use a more modern validation package like https://github.com/go-playground/validator/ ?

Please correct me, I may have missed some important technical details of this choice.

Thanks.