How to validate string is either nil or longer than 0

[evgeniy-ivlev-archie]

Hello.

I need the string to either be nil or longer than 0.

func init() {
	validate = validator.New()

	validate.RegisterValidation("nil_or_non_empty_string", func(fl validator.FieldLevel) bool {
                // NOTHING WORKS HERE
                // I CANNOT DETECT NIL POINTER
         
		v := fl.Field()

		if v.IsNil() {
			return true
		}

		if v.String() != "" {
			return true
		}

		return false
        }
}

Can someone please help me?
Thanks kindly

[nodivbyzero]

@evgeniy-ivlev-archie Here you go:

package main

import (
	"fmt"

	"github.com/go-playground/validator/v10"
)

// User represents user data with validation tags.
type User struct {
	Username string  `json:"username" validate:"required,min=3"`
	Email    string  `json:"email" validate:"required,email"`
	NickName *string `json:"nickname" validate:"omitnil,min=1"`
}

// validateUser runs validation and prints the result.
func validateUser(v *validator.Validate, user User, caseName string) {
	if err := v.Struct(user); err != nil {
		fmt.Printf("[%s] ❌ Validation failed: %v\n", caseName, err)
	} else {
		fmt.Printf("[%s] ✅ Validation passed\n", caseName)
	}
}

func main() {
	v := validator.New(validator.WithRequiredStructEnabled())

	testCases := []struct {
		name string
		user User
	}{
		{
			name: "Valid user without nickname",
			user: User{
				Username: "test",
				Email:    "test@test.com",
			},
		},
		{
			name: "Invalid user with empty nickname",
			user: func() User {
				nick := ""
				return User{
					Username: "test",
					Email:    "test@test.com",
					NickName: &nick,
				}
			}(),
		},
		{
			name: "Valid user with nickname",
			user: func() User {
				nick := "n"
				return User{
					Username: "test",
					Email:    "test@test.com",
					NickName: &nick,
				}
			}(),
		},
	}

	for _, tc := range testCases {
		validateUser(v, tc.user, tc.name)
	}
}

https://go.dev/play/p/d-dgMHbMt3a