using System;

using System.Text.RegularExpressions;

public class Program

    {

        public static void Main(string[] args)

        {

            Console.WriteLine(ZipZap("zipXzap"));

            Console.WriteLine(ZipZap("zopzop"));

            Console.WriteLine(ZipZap("zzzopzop"));

            Console.ReadLine();

        }

​

        /* Look for patterns like "zip" and "zap" in the string -- length-3, starting with 'z' and ending with 'p'. 

         * Return a string where for all such words, the middle letter is gone, so "zipXzap" yields "zpXzp". */

        public static string ZipZap(string str)

        {

            Regex rx = new Regex(@"(z)(.{1})(p)");

            return rx.Replace(str, match => match.ToString().Remove(1,1));

        }

    }