C#-Code

using System;
using System.Collections;
using System.Collections.Generic;
using System.Linq;
using System.Net;
using System.Text;
using System.Text.RegularExpressions;
using System.Web.Script.Serialization;//benötigt Verweis auf System.Web.Extensions

namespace ConsoleZipCodeValidation
{
    class Program
    {
        static void Main(string[] args)
        {
            Console.Write("Geben Sie eine deutsche PLZ ein: ");
            string plz = Console.ReadLine();

            //Mit Webservice
            Console.Write("In welchen Ländern soll gesucht werden? (durch Komma trennen): ");
            string[] searchIn = Console.ReadLine().Split(',');
            var foundCities = WebserviceZipCodeValidation.ValidateZipCode(plz, searchIn);
            if (foundCities.Count() == 0)
            {
                Console.WriteLine("Die PLZ konnte nicht gefunden werden");
            }
            else
            {
                Console.WriteLine("Folgende Treffer gefunden:");

                foreach (var city in foundCities)
                {
                    Console.WriteLine(city);
                }
            }
            //

            //Mit Regex
            //if (SimpleZipCodeValidation.IsGermanZipCode(plz))
            //{
            //    Console.WriteLine("Die eingebene PLZ ist eine deutsche PLZ");
            //}
            //else
            //{
            //    Console.WriteLine("Die eingebene PLZ ist keine deutsche PLZ");
            //}

            Console.ReadLine();
        }
    }

    static class SimpleZipCodeValidation
    {
        const string onlyNumbers = "^[0-9]+$";
        public static bool IsGermanZipCode(string zipCode)
        {
            return zipCode.Length == 5 && Regex.IsMatch(zipCode, onlyNumbers);
        }
    }

    class WebserviceZipCodeValidation
    {
        const string webserviceUrl = "http://api.zippopotam.us/{0}/{1}";//0 = Landkennung (de, at, ch, ...), 1 = PLZ

        public static IEnumerable<City> ValidateZipCode(string zipCode, params string[] searchInCountrys)
        {
            foreach (string s in searchInCountrys)
            {
                string city, country;
                if (ValidateZipCode(zipCode, out country, out city, s))
                {
                    yield return new City() { Cityname = city, Country = country, ZipCode = zipCode };
                }
            }
        }

        public static bool ValidateZipCode(string zipCode, out string country, out string city, string searchInCountry)
        {
            try
            {
                WebClient client = new WebClient();
                string url = CreateRequestString(searchInCountry, zipCode);
                string json = Encoding.UTF8.GetString(client.DownloadData(url));

                Dictionary<string, object> filters = new JavaScriptSerializer().Deserialize<Dictionary<string, object>>(json);
                country = filters["country"].ToString();
                object[] places = ((ArrayList)filters["places"]).ToArray();
                if (places.Length == 1)
                {
                    Dictionary<string, object> placesDic = (Dictionary<string, object>)places[0];
                    city = placesDic["place name"].ToString();
                    return true;
                }
                else
                {
                    country = "";
                    city = "";
                    return false;
                }

            }
            catch (Exception)
            {
                country = "";
                city = "";
                return false;
            }
        }

        static string CreateRequestString(string country, string zipCode)
        {
            return string.Format(webserviceUrl, country, zipCode);
        }
    }

    class City
    {
        public string Cityname { get; set; }
        public string Country { get; set; }
        public string ZipCode { get; set; }

        public override string ToString()
        {
            return $"{ZipCode} {Cityname}, {Country}";
        }
    }
}

C#-Code

using System;
using System.Text.RegularExpressions;

namespace RegexPlzValidator
{
    class Program
    {
        static void Main(string[] args)
        {
            if (args.Length == 1)
            {
                string isPlzValide = (validatePlz(args[0]) ? "valide" : "nicht valide.");
                Console.WriteLine("Die eingegebene PLZ \"{0}\" ist {1}.", args[0], isPlzValide);
            }
            else
            {
                Console.WriteLine("Parameter fehler:\nBitte einen einzigen Parameter als PLZ uebergeben.");
            }

            Console.ReadKey();
        }

        private static bool validatePlz(string plz)
        {
            return new Regex("^[0-9]{5}$").IsMatch(plz);
        }
    }
}