using System;

using System.Text;

using System.Text.RegularExpressions;

using System.Globalization;

using System.Linq;

using System.Security.Cryptography;

public class Program

{

    private static SHA256 shaEncryptor = SHA256.Create();

    private static string CalculateSha2Hash(string input)

    {

        var utf8Encoder = new UTF8Encoding(true);

        var hashedBytes = shaEncryptor.ComputeHash(utf8Encoder.GetBytes(input));

        return hashedBytes.Aggregate(string.Empty, (current, item) => current + item.ToString("X2", CultureInfo.InvariantCulture)).ToLowerInvariant();

    }

    public static void Main()

    {

        string message = "hello 8:narinderberi this is a message";

        string pattern = @"(8:[^: ]+)( |$)";

        MatchCollection matches = Regex.Matches(message, pattern);

        foreach (Match match in matches)

        {

            GroupCollection groups = match.Groups;

            Console.WriteLine("User Id Captured: <" + groups[1] + ">");

            //Replace

            message = message.Substring(0, groups[1].Index) + CalculateSha2Hash(groups[1].ToString()) + message.Substring(groups[1].Index + groups[1].Length);

        }

        Console.WriteLine(message);

    }

}