using System;

using System.Text.RegularExpressions;

public class Program

{

    public static void Main()

    {

        string[] values = 

        {

            "MMM\\'\\'yy",

            "MMM\\'s\\'yy",

            "\\'MMM 'yy",

            "'MMM'yy'",

            "MMM''yy",

            "''''"

        };

        Regex _matchApostropheReg =  new Regex("(?<!\\\\)'");

        foreach(var value in values) {

            Console.WriteLine("---------");

            Console.WriteLine(value);

            Console.WriteLine(_matchApostropheReg.Replace(value, "\\'"));

        }

    }

}