go get github.com/go-playground/validator/v10

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

package main
import (
	"fmt"
	"github.com/go-playground/validator/v10"
)
type User struct {
	Name string `validate:"min=6,max=10"`
	Age  int    `validate:"min=1,max=100"`
}
func main() {
	validate := validator.New()
	u1 := User{Name: "lidajun", Age: 18}
	err := validate.Struct(u1)
	fmt.Println(err)
	u2 := User{Name: "dj", Age: 101}
	err = validate.Struct(u2)
	fmt.Println(err)
}

func validate(u User) bool {
	if u.Age < 1 || u.Age > 100 {
		return false
	}
	if len(u.Name) < 6 || len(u.Name) > 10 {
		return false
	}
	return true
}

package main
import (
	"fmt"
	"github.com/go-playground/validator/v10"
)
// User contains user information
type User struct {
	FirstName      string     `validate:"required"`
	LastName       string     `validate:"required"`
	Age            uint8      `validate:"gte=0,lte=130"`
	Email          string     `validate:"required,email"`
	FavouriteColor string     `validate:"iscolor"`                // alias for 'hexcolor|rgb|rgba|hsl|hsla'
	Addresses      []*Address `validate:"required,dive,required"` // a person can have a home and cottage...
}
// Address houses a users address information
type Address struct {
	Street string `validate:"required"`
	City   string `validate:"required"`
	Planet string `validate:"required"`
	Phone  string `validate:"required"`
}
// use a single instance of Validate, it caches struct info
var validate *validator.Validate
func main() {
	validate = validator.New()
	validateStruct()
	validateVariable()
}
func validateStruct() {
	address := &Address{
		Street: "Eavesdown Docks",
		Planet: "Persphone",
		Phone:  "none",
	}
	user := &User{
		FirstName:      "Badger",
		LastName:       "Smith",
		Age:            135,
		Email:          "Badger.Smith@gmail.com",
		FavouriteColor: "#000-",
		Addresses:      []*Address{address},
	}
	// returns nil or ValidationErrors ( []FieldError )
	err := validate.Struct(user)
	if err != nil {
		// this check is only needed when your code could produce
		// an invalid value for validation such as interface with nil
		// value most including myself do not usually have code like this.
		if _, ok := err.(*validator.InvalidValidationError); ok {
			fmt.Println(err)
			return
		}
		for _, err := range err.(validator.ValidationErrors) {
			fmt.Println(err.Namespace())
			fmt.Println(err.Field())
			fmt.Println(err.StructNamespace())
			fmt.Println(err.StructField())
			fmt.Println(err.Tag())
			fmt.Println(err.ActualTag())
			fmt.Println(err.Kind())
			fmt.Println(err.Type())
			fmt.Println(err.Value())
			fmt.Println(err.Param())
			fmt.Println()
		}
		// from here you can create your own error messages in whatever language you wish
		return
	}
	// save user to database
}
func validateVariable() {
	myEmail := "joeybloggs.gmail.com"
	errs := validate.Var(myEmail, "required,email")
	if errs != nil {
		fmt.Println(errs) // output: Key: "" Error:Field validation for "" failed on the "email" tag
		return
	}
	// email ok, move on
}

// User contains user information
type User struct {
	FirstName      string     `validate:"required"`
	LastName       string     `validate:"required"`
	Age            uint8      `validate:"gte=0,lte=130"`
	Email          string     `validate:"required,email"`
	FavouriteColor string     `validate:"iscolor"`                // alias for 'hexcolor|rgb|rgba|hsl|hsla'
	Addresses      []*Address `validate:"required,dive,required"` // a person can have a home and cottage...
}
// Address houses a users address information
type Address struct {
	Street string `validate:"required"`
	City   string `validate:"required"`
	Planet string `validate:"required"`
	Phone  string `validate:"required"`
}

[][]string with validation tag "gt=0,dive,len=1,dive,required"
// gt=0 will be applied to []
// len=1 will be applied to []string
// required will be applied to string

[][]string with validation tag "gt=0,dive,dive,required"
// gt=0 will be applied to []
// []string will be spared validation
// required will be applied to string

err := validate.Struct(mystruct)
validationErrors := err.(validator.ValidationErrors)

// ValidationErrors is an array of FieldError's
// for use in custom error messages post validation.
type ValidationErrors []FieldError
// Error is intended for use in development + debugging and not intended to be a production error message.
// It allows ValidationErrors to subscribe to the Error interface.
// All information to create an error message specific to your application is contained within
// the FieldError found within the ValidationErrors array
func (ve ValidationErrors) Error() string {
	buff := bytes.NewBufferString("")
	var fe *fieldError
	for i := 0; i < len(ve); i++ {
		fe = ve[i].(*fieldError)
		buff.WriteString(fe.Error())
		buff.WriteString("\n")
	}
	return strings.TrimSpace(buff.String())
}

// FieldError contains all functions to get error details
type FieldError interface {
	// Tag returns the validation tag that failed. if the
	// validation was an alias, this will return the
	// alias name and not the underlying tag that failed.
	//
	// eg. alias "iscolor": "hexcolor|rgb|rgba|hsl|hsla"
	// will return "iscolor"
	Tag() string
	// ActualTag returns the validation tag that failed, even if an
	// alias the actual tag within the alias will be returned.
	// If an 'or' validation fails the entire or will be returned.
	//
	// eg. alias "iscolor": "hexcolor|rgb|rgba|hsl|hsla"
	// will return "hexcolor|rgb|rgba|hsl|hsla"
	ActualTag() string
	// Namespace returns the namespace for the field error, with the tag
	// name taking precedence over the field's actual name.
	//
	// eg. JSON name "User.fname"
	//
	// See StructNamespace() for a version that returns actual names.
	//
	// NOTE: this field can be blank when validating a single primitive field
	// using validate.Field(...) as there is no way to extract it's name
	Namespace() string
	// StructNamespace returns the namespace for the field error, with the field's
	// actual name.
	//
	// eq. "User.FirstName" see Namespace for comparison
	//
	// NOTE: this field can be blank when validating a single primitive field
	// using validate.Field(...) as there is no way to extract its name
	StructNamespace() string
	// Field returns the fields name with the tag name taking precedence over the
	// field's actual name.
	//
	// eq. JSON name "fname"
	// see StructField for comparison
	Field() string
	// StructField returns the field's actual name from the struct, when able to determine.
	//
	// eq.  "FirstName"
	// see Field for comparison
	StructField() string
	// Value returns the actual field's value in case needed for creating the error
	// message
	Value() interface{}
	// Param returns the param value, in string form for comparison; this will also
	// help with generating an error message
	Param() string
	// Kind returns the Field's reflect Kind
	//
	// eg. time.Time's kind is a struct
	Kind() reflect.Kind
	// Type returns the Field's reflect Type
	//
	// eg. time.Time's type is time.Time
	Type() reflect.Type
	// Translate returns the FieldError's translated error
	// from the provided 'ut.Translator' and registered 'TranslationFunc'
	//
	// NOTE: if no registered translator can be found it returns the same as
	// calling fe.Error()
	Translate(ut ut.Translator) string
	// Error returns the FieldError's message
	Error() string
}