sortinfo = sortinfo.Remove(0, 500);
sortinfo = new string(sortinfo.SkipWhile(c => !char.IsDigit(c)).ToArray());

class Program
{
    static void Main(string[] args)
    {
        string str;
        str=TrimSortInfo("23:value,ord4er24:valueag4ain,order6again15:value,order"); // breaking value
        //str = TrimSortInfo("23:value,order24:valueagain,orderagain15:value,order"); //working value
        Console.WriteLine(str);
        Console.ReadLine();

    }

    static string TrimSortInfo(string sortinfo)
    {
        if (sortinfo.Length > 15)
        {
            sortinfo = sortinfo.Remove(0, 15);
            sortinfo = new string(sortinfo.SkipWhile(c => !char.IsDigit(c))
                         .ToArray());
            return sortinfo;
        }
        return sortinfo;
    }
}