C#-Code

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

namespace trainYourProgrammer
{
	class MainClass
	{
		private static bool IBANvalid(string IBAN)
		{
			IBAN = IBAN.Replace (" ", "").Replace ("-", "");
			Dictionary<string, int> countryLength = new Dictionary<string, int> ();
			countryLength.Add ("DE", 20); countryLength.Add ("AT", 18); countryLength.Add ("CH", 19);
			string country = IBAN.Substring (0, 2);
			int length;
			if (!countryLength.TryGetValue(country, out length))
				return false; //Land ist unbekannt
			if (Regex.IsMatch (IBAN, "^[A-Z]{2}[0-9]{2}[A-Za-z0-9]{" + length + "}$"))
				return false;
			string IBANcheck = IBAN.Substring (4) + IBAN.Substring (0, 4);
			for(char c = 'A'; c <= 'Z'; c++)
				IBANcheck = IBANcheck.Replace(c.ToString(), ((int)c - 'A' + 10).ToString());
			while (IBANcheck.Length > 8) { //die Zahl ist zu groß for long, zerlegen in mehrere Schritte
				int tmp = int.Parse (IBANcheck.Substring (0, 8));
				IBANcheck = (tmp % 97) + IBANcheck.Substring (8);
			}
			return (int.Parse (IBANcheck)) % 97 == 1;
		}

		private static void test()
		{
			string[] valid = {"DE07 1234 1234 1234 1234 12", "DE07-1234-1234-1234-1234-12", "DE07123412341234123412", "AT84 1234 1234 1234 1234", "CH87 1234 1234 1234 1234 1"};
			string[] invalid = {"DE19 1234 1234 1234 1234 12", "DE07 1234 1234 1234 1234 123", "AT84 1234 1234 1234 134"};
			foreach (string s in valid)
				if (!IBANvalid(s)) Console.WriteLine("Fehler bei " + s);
			foreach (string s in invalid)
				if (IBANvalid(s)) Console.WriteLine("Fehler bei " + s);
		}

		static void Main(string[] args)
		{
			test ();
			while (true) {
				Console.Write ("IBAN eingeben: ");
				string iban = Console.ReadLine ();
				Console.WriteLine ("Die Addresse ist " + (IBANvalid(iban)? "" : "nicht ") + "gültig\n");
			}
		}
	}
}