[RegularExpression(@"^[_a-zA-Z0-9]{6.24}?$", ErrorMessage = "{0}只能包含字母/数字/下划线")]

    public string Pwd { get; set; }


string pwd = "123456aa";
           Regex regex2 = new Regex("^[_a-zA-Z0-9]{6,24}$");
           Match match = regex2.Match(pwd);
           Console.WriteLine(match.Success && match.Index == 0);
           Console.WriteLine(match.Length == pwd.Length);

    public override bool IsValid(object value)
    {
      this.SetupRegex();
      string input = Convert.ToString(value, (IFormatProvider) CultureInfo.CurrentCulture);
      if (string.IsNullOrEmpty(input))
        return true;
      Match match = this.Regex.Match(input);
      if (match.Success && match.Index == 0)
        return match.Length == input.Length;
      return false;
    }

        private void SetupRegex()
        {
            if (this.Regex != null)
                return;
this.Regex = new Regex("^[_a-zA-Z0-9]{6,24}?$");
        }