code: func ValidateFast(email string) error {
  	if len(email) < 6 || len(email) > 254 {
  		return ErrInvalidFormat
  	}
  
  	at := strings.LastIndex(email, "@")
  	if at <= 0 || at > len(email)-3 {
  		return ErrInvalidFormat
  	}
  
  	user := email[:at]
  	host := email[at+1:]
  
  	if len(user) > 64 {
  		return ErrInvalidFormat
  	}
  	if userDotRegexp.MatchString(user) || !userRegexp.MatchString(user) || !hostRegexp.MatchString(host) {
  		return ErrInvalidFormat
  	}
  
  	return nil
  }
docs: // ValidateFast checks format of a given email.