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//Title of this code
//Rextester.Program.Main is the entry point for your code. Don't change it.
//Compiler version 4.0.30319.17929 for Microsoft (R) .NET Framework 4.5

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text.RegularExpressions;

namespace Rextester
{
    public class Program
    {
        public static void Main(string[] args)
        {
            var items = new List<Description>()
            {
                new Description(){Desc3="test test test Card#1234"},
                new Description(){Desc3="dsd dsftest Card#1234"},
                new Description(){Desc3="jjjjj iiiiii kkkk Card#1234"},
                new Description(){Desc3="ujeuejduejtest test Card#9999"},
                new Description(){Desc3="2323fseff dsftest Card#9999"},
                new Description(){Desc3="sdfsd fsdsdf kkkk Card#9999"}
            };
            
            var result = items.GroupBy(i => {
                var idx = i.Desc3.IndexOf("Card#");
                if(idx == -1)
                    return null;
                
                return i.Desc3.Substring(idx+5);
            });
            
            foreach(var r in result)
            {
                Console.WriteLine(r.Key);
                foreach(var item in r)
                {
                    Console.WriteLine("\t{0}",item.Desc3);
                }
            }
        }
    }
    
     public class Description
    {
         public Description(){}
    
    
        public string Desc1 { get; set; }
        public string Desc2 { get; set; }
        public string Desc3 { get; set; }
        //public DayMonthYear OriginalDate { get; set; }
        //public DayMonthYear PostedDate { get; set; }
    
    }
}

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Compilation time: 0.08 sec, absolute running time: 0.08 sec, cpu time: 0.06 sec, average memory usage: 14 Mb, average nr of threads: 4