using System;

using System.Text.RegularExpressions;

​

public class Program

{

    public static void Main()

    {

        var input = "MyApp v2020.8.3.1.exe";

        var m = Regex.Match(input, @"(\d+)\.(\d+)\.(\d+)\.(\d+)");

        var result = $"{m.Groups[1]}.{m.Groups[2]}.{m.Groups[3]}-r{m.Groups[4]}";

        Console.WriteLine(result);

    }

}