code: func isISBN13(fl FieldLevel) bool {
  
  	s := strings.Replace(strings.Replace(fl.Field().String(), "-", "", 4), " ", "", 4)
  
  	if !iSBN13Regex.MatchString(s) {
  		return false
  	}
  
  	var checksum int32
  	var i int32
  
  	factor := []int32{1, 3}
  
  	for i = 0; i < 12; i++ {
  		checksum += factor[i%2] * int32(s[i]-'0')
  	}
  
  	return (int32(s[12]-'0'))-((10-(checksum%10))%10) == 0
  }
docs: // IsISBN13 is the validation function for validating if the field's value is a valid v13 ISBN.