xml-ltt.cs

            String abosulteFilePath = @"C:\Users\KNG HOLDY\Desktop\test.xml";

            String timePattern = "<Cube time=\"(\\d{4})-(\\d{2})-(\\d{2})\">"; //<Cube time="2020-03-16">
            String cubePattern = "<Cube currency=\"(.*?)\" rate=\"(.*?)\"/>"; // <Cube currency="JPY" rate="117.12"/>

            Regex timeRegexp = new Regex(timePattern, RegexOptions.IgnoreCase);
            Regex cubeRegexp = new Regex(cubePattern, RegexOptions.IgnoreCase);


            using (var sr = new StreamReader(abosulteFilePath))
            {
                string xmlString = sr.ReadToEnd();

                //Trim XML-File

                int firstCubeOccurnace = xmlString.IndexOf("<Cube>") + "<Cube>".Length;
                int firstCubeEndOccurnace = xmlString.IndexOf("</Cube>");

                string firstCube = xmlString.Remove(firstCubeEndOccurnace).Remove(0, firstCubeOccurnace);

                //Find Date of First Cube Occurance => <Cube time="2020-03-16">

                var timeMatch = timeRegexp.Match(firstCube);
                if (timeMatch.Success)
                {
                    GroupCollection resultGroup = timeMatch.Groups;

                    if(resultGroup.Count == 4)
                    {
                        String year = resultGroup[1].Value;
                        String month = resultGroup[2].Value;
                        String day = resultGroup[2].Value;                        

                        Console.WriteLine(String.Format("Date: {0}-{1}-{2}", year, month, day));
                    }

                }
                else
                {
                    //TODO
                    Console.WriteLine("NO MATCH");
                }

                //Find alle Cube's inside Parent Cube
                foreach (Match cubeMatch in cubeRegexp.Matches(firstCube))
                {
                    GroupCollection resultGroup = cubeMatch.Groups;

                    if(resultGroup.Count != 3)
                    {
                        //TODO exception
                    }

                    String currency = resultGroup[1].Value;
                    String rate = resultGroup[2].Value;

                    Console.WriteLine(String.Format("Date: currency = {0}, rate = {1}", currency, rate));
                }
            }