validate number of digits by golang validator

main.go

package main

import (
	"fmt"
	"os"
	"strconv"

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

func validateNumberOfDigit(fl validator.FieldLevel) bool {
	field := fl.Field()
	param, err := strconv.Atoi(fl.Param())
	if err != nil {
		panic(err.Error())
	}

	v := field.Int()
	if v < 0 {
		panic("negative number")
	}

	n := 0
	for ; v > 0; v /= 10 {
		n += 1
	}

	return n == param
}

type userCred struct {
	Email    string `json:"email" validate:"required,email"`
	Password string `json:"password" validate:"required,min=6,max=15"`
	Phone    int    `json:"phone" validate:"required,numeric,int_len=8"`
}

func main() {
	u := userCred{
		Email:    "xxxx@yyyy.zz",
		Password: "abcdefghi",
		Phone:    8,
	}

	validate := *validator.New()
	err := validate.RegisterValidation("int_len", validateNumberOfDigit)
	if err != nil {
		panic(err)
	}

	err = validate.Struct(&u)
	if err != nil {
		fmt.Println(err)
		os.Exit(1)
	}

	fmt.Println("ok")
}