Axiomatik, thanks a lot!

With your regex my result in Judge is also 80/100. :(

With my new regex with some corrections in the begining and in the end (@"@(?<planet>[A-za-z]+)[^@!:>-]*:(?<population>[0-9]+)[^@!:>-]*!(?<attack>[A|D])![^@!:>-]*->(?<soldierCount>[0-9]+)") the result is also 80/100.

From my understanding [AD] and [A|D] are doing the same and I don't need "\" to escape the "-", because I placed "-" at the end.

Iam very confused and worried with these regular expressions.

The last three exercises (Star Enigma, Nether Realms and Extract Emails) are a bit tricky and I also had difficulties with them. More importantly, start training with the older exams that are available and watch exam preparation videos to see how they are being solved. Unfortunately, C# Advanced won't be easier and you just have to keep up with the new material.

Best,

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

namespace starEnigmaFinal
{
    class Program
    {
        static void Main(string[] args)
        {
            int messages = int.Parse(Console.ReadLine());
            string fullPattern = @"@(?<name>[A-Za-z]+)[^@\-!:>]*:(?<population>\d+)[^@\-!:>]*!(?<type>A|D)![^@\-!:>]*->(?<soldierCount>\d+)";
            string starPattern = @"[starSTAR]";

            List<string> attackedPlanets = new List<string>();
            List<string> destroyedPlanets = new List<string>();

            for (int i = 0; i < messages; i++)
            {
                string message = Console.ReadLine();
                MatchCollection starMatches = Regex.Matches(message, starPattern);
                int countLetters = starMatches.Count;

                string decryptedMessage = string.Empty;

                foreach (var letter in message)
                {
                    decryptedMessage += (char)(letter - countLetters);
                }

                Match planetValid = Regex.Match(decryptedMessage, fullPattern);
                if (planetValid.Success)
                {
                    string planetName = planetValid.Groups["name"].Value;
                    string typeAttack = planetValid.Groups["type"].Value;

                    if (typeAttack == "A")
                    {
                        attackedPlanets.Add(planetName);
                    }
                    else if (typeAttack == "D")
                    {
                        destroyedPlanets.Add(planetName);
                    }
                }
            }

            Console.WriteLine($"Attacked planets: {attackedPlanets.Count}");

            if (attackedPlanets.Any())
            {
                attackedPlanets.Sort();
                foreach (var planet in attackedPlanets)
                {
                    Console.WriteLine($"-> {planet}");
                }
            }

            Console.WriteLine($"Destroyed planets: {destroyedPlanets.Count}");
            if (destroyedPlanets.Any())
            {
                destroyedPlanets.Sort();
                foreach (var planet in destroyedPlanets)
                {
                    Console.WriteLine($"-> {planet}");
                }
            }
        }
    }
}

At last... thanks to you I have 100/100. :) https://pastebin.com/NsvW6D7c

1) Judge is serching for  "[A|D]" and it doesn't matter that [AD] and [A|D] are doing the same.

2) Judge dislikes this check up " if (regexForKey.IsMatch(message))
                {
                    int key = matches.Count;
                    string decryptedMessage = string.Empty;
                    foreach (char symbol in message)
                    {
                        int decryptedSymbolInt = symbol - key;
                        char decryptedSymbol = (char)decryptedSymbolInt;
                        decryptedMessage += decryptedSymbol;
                    } "

Should be whitout if condition:

                  var matches = regexForKey.Matches(message);

                    int key = matches.Count;
                    string decryptedMessage = string.Empty;
                    foreach (char symbol in message)
                    {
                        int decryptedSymbolInt = symbol - key;
                        char decryptedSymbol = (char)decryptedSymbolInt;
                        decryptedMessage += decryptedSymbol;

                     }

For second if\else condition and the both are valid: "

MatchCollection newMatches = regexForDecrypt.Matches(decryptedMessage); with forech loop", like in my code.

"Match newMatch=regexForDecrypt.Match(decryptedMessage); whitout forech loop", like in your code.

I think your way is better than mine. If I don't need I collection it haven't  reason to used it and to used forech loop for every match. :)

And thanks a lot for your advice. By the way  I plan to spend 6 months with C# Advanced, 2 months with High Quality Code and design patterns : https://softuni.bg/trainings/2706/high-quality-code-december-2019 , 3 months with Data Structure (fundamentals and Advanced) and 3 months with Algorithms (fundamentals and Advanced), before I go to DataBases and Web. After that I will start with HTML, CSS , JavaScript and Agile course... and do my best to not give up.

All the best!

Eli