using System;

using System.Linq;

public class Program

{

    public static void Main()

    {

                var backendString_ExternalTransactionId = "OCS,OI:MP2220216156,AU:086133";

                var correlatorId = "";

                var auId = "";

                if (!string.IsNullOrEmpty(backendString_ExternalTransactionId))

                {

                    var tRefSplit = backendString_ExternalTransactionId.Split(',');

​

​

                    if (tRefSplit.Length > 1)

                    {

                        var oi = tRefSplit.Skip(1).FirstOrDefault().Split(':');

                        correlatorId = oi.TakeLast(1).FirstOrDefault();

                    }

​

                    if (tRefSplit.Length > 2)

                    {

                        var au = tRefSplit.Skip(2).FirstOrDefault().Split(':');

                        auId = au.TakeLast(1).FirstOrDefault();

                    }

                }

                Console.WriteLine(correlatorId);

    }

}