Расшифруйте число строк в алфавите на C#

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Dear All,
I want to convert number string entered in a textbox into integer array, and later on getting alphabet according index value of character array, and value of integer array.
Although, by using lambda expression, I can easily achieve that. But I want to achieve the same without lambda expression, and alternative regular expression for the same. 
Note:- In given below code, without lambda expression, if I enter “0AB12”, then it will print AABBC, instead of I want to ignore alphabet, and just want to decode in integer. While with lambda expression, it gives me perfect answer- ABC

Please help, it was asked me in a recent interview as a fresher.

protected void btn_Decode_Click(object sender, EventArgs e)
    {
        char[] arr = new char[] {'A','B','C','D','E','F','G','H','I','J','K','L','M','N','O','P','Q','R','S','T','U','V','W','X','Y','Z' };
        string result = string.Empty;
        //string input = '0231';
        var myString = txt_Number.Text;
        int myInt;
        int[] num_arr = new int[myString.Length];
        for (int i = 0; i <myString.Length; i++)
        {
             if (int.TryParse(myString.Substring(i,1),out myInt))
            {
              num_arr[i] = int.Parse(myString.Substring(i, 1));
            }
            

        }
        //int myInt;


        //var num_arr = myString.ToCharArray().Where(x => int.TryParse(x.ToString(), out myInt)).Select(x => int.Parse(x.ToString())).ToArray();
        //int[] num_arr = new int[] { 0, 2, 3, 1 };


        for (int i = 0; i < num_arr.Length; i++)
        {
            result = result + arr[num_arr[i]];
        }

        Response.Write(result); 

    }

Что я уже пробовал:

protected void btn_Decode_Click(object sender, EventArgs e)
    {
        char[] arr = new char[] {'A','B','C','D','E','F','G','H','I','J','K','L','M','N','O','P','Q','R','S','T','U','V','W','X','Y','Z' };
        string result = string.Empty;
        //string input = '0231';
        var myString = txt_Number.Text;
        int myInt;
        int[] num_arr = new int[myString.Length];
        for (int i = 0; i <myString.Length; i++)
        {
             if (int.TryParse(myString.Substring(i,1),out myInt))
            {
              num_arr[i] = int.Parse(myString.Substring(i, 1));
            }
            

        }
        //int myInt;


        //var num_arr = myString.ToCharArray().Where(x => int.TryParse(x.ToString(), out myInt)).Select(x => int.Parse(x.ToString())).ToArray();
        //int[] num_arr = new int[] { 0, 2, 3, 1 };


        for (int i = 0; i < num_arr.Length; i++)
        {
            result = result + arr[num_arr[i]];
        }

        Response.Write(result); 

    }

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