Formal grammar for uni

using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Runtime.InteropServices;
using System.Text;
using System.Threading.Tasks;

namespace MacAuto1_CTest
{
    internal class Program
    {
        static StringBuilder ReplaceChar(StringBuilder S, int num)
        {
            switch (num)
            {
                case 1: S.Replace("0", "aSSa"); break;
                case 2: S.Replace("0", "bSb"); break;
                case 3: S.Replace("0", "cAb"); break;
                case 4: S.Replace("0", "Aa"); break;
                case 5: S.Replace("0", "Ba"); break;
                case 6: S.Replace("0", "e"); break;
                case 7: S.Replace("0", "bB"); break;
                case 8: S.Replace("0", "aA"); break;
                default: Console.WriteLine("error"); return S;
            }
            return S;
        }

        static StringBuilder grammar_outL(StringBuilder S, StringBuilder sequence)
        {
            //Random rnd = new Random();
            byte input_var = 0;
            //int random_num = rnd.Next(1, 4);
            //Console.Write(" -> " + S);
            for (int i = 0; i < S.Length; i++)
            {
                switch (S[i])
                {
                    case 'S':
                        //random_num = rnd.Next(1, 4);
                        Console.WriteLine("Choose a rule: ");
                        Console.WriteLine("1. S -> aSSa\n2. S -> bSb\n3. S -> cAb");
                        do
                        {
                            input_var = Convert.ToByte(Console.ReadLine());
                        }
                        while (input_var < 1 || input_var > 3);
                        sequence.Append(input_var);
                        S[i] = '0';
                        ReplaceChar(S, input_var);
                        i = 0;
                        Console.WriteLine(S);
                        break;

                    case 'A':
                        //random_num = rnd.Next(4, 7);
                        Console.WriteLine("4. A -> Aa\n5. A -> Ba\n6. A -> epsilon");
                        Console.WriteLine("Choose a rule: ");
                        do
                        {
                            input_var = Convert.ToByte(Console.ReadLine());
                        }
                        while (input_var < 4 || input_var > 7);
                        sequence.Append(input_var);
                        S[i] = '0';
                        ReplaceChar(S, input_var);
                        i = 0;
                        Console.WriteLine(S);
                        break;

                    case 'B':
                        //random_num = rnd.Next(7, 9);
                        Console.WriteLine("7. B -> bB\n8. B -> aA");
                        Console.WriteLine("Choose a rule: ");
                        do
                        {
                            input_var = Convert.ToByte(Console.ReadLine());
                        }
                        while (input_var < 7 || input_var > 8);
                        sequence.Append(input_var);
                        S[i] = '0';
                        ReplaceChar(S, input_var);
                        i = 0;
                        Console.WriteLine(S);
                        break;
                    default: break;
                }
            }
            S.Replace("e", "");
            return S;
        }

        static StringBuilder grammar_outR(StringBuilder S, StringBuilder sequence)
        {
            //Random rnd = new Random();
            byte input_var = 0;
            //int random_num = rnd.Next(1, 4);
            //Console.Write(" -> " + S);
            Console.WriteLine("\nS");
            for (int i = S.Length - 1; i >= 0; i--)
            {
                switch (S[i])
                {
                    case 'S':
                        //random_num = rnd.Next(1, 4);
                        Console.WriteLine("Choose a rule: ");
                        Console.WriteLine("1. S -> aSSa\n2. S -> bSb\n3. S -> cAb");
                        do
                        {
                            input_var = Convert.ToByte(Console.ReadLine());
                        }
                        while (input_var < 1 || input_var > 3);
                        sequence.Append(input_var);
                        S[i] = '0';
                        ReplaceChar(S, input_var);
                        i = S.Length - 1;
                        Console.WriteLine(S);
                        break;

                    case 'A':
                        //random_num = rnd.Next(4, 7);
                        Console.WriteLine("4. A -> Aa\n5. A -> Ba\n6. A -> epsilon");
                        Console.WriteLine("Choose a rule: ");
                        do
                        {
                            input_var = Convert.ToByte(Console.ReadLine());
                        }
                        while (input_var < 4 || input_var > 6);
                        sequence.Append(input_var);
                        S[i] = '0';
                        if (input_var == 6)
                        {
                            S.Remove(i, 1);
                        }
                        else
                            ReplaceChar(S, input_var);
                        i = S.Length - 1;
                        Console.WriteLine(S);
                        break;

                    case 'B':
                        //random_num = rnd.Next(7, 9);
                        Console.WriteLine("7. B -> bB\n8. B -> aA");
                        Console.WriteLine("Choose a rule: ");
                        do
                        {
                            input_var = Convert.ToByte(Console.ReadLine());
                        }
                        while (input_var < 7 || input_var > 8);
                        sequence.Append(input_var);
                        S[i] = '0';
                        ReplaceChar(S, input_var);
                        i = S.Length - 1;
                        Console.WriteLine(S);
                        break;
                    default: break;
                }
            }
            S.Replace("e", "");
            return S;
        }

        static string Linear_Tree(StringBuilder sequence)
        {
            string _Linear_Tree = "S()";
            var for_replacing = new StringBuilder("");
            for (int i = 0; i < sequence.Length; i++)
            {
                for_replacing.Clear();
                for_replacing.Append(0);
                for (int j = 0; j < _Linear_Tree.Length; j++)
                {
                    if (j != _Linear_Tree.Length - 1 && _Linear_Tree[j + 1] == ')' && _Linear_Tree[j] == '(')
                    {
                        _Linear_Tree = _Linear_Tree.Insert(j + 1, Convert.ToString(ReplaceChar(for_replacing, Convert.ToInt32(sequence[i]) - 48)));

                    }

                    if ((_Linear_Tree[j] == 'S' || _Linear_Tree[j] == 'A' || _Linear_Tree[j] == 'B') && _Linear_Tree[j + 1] != '(')
                    {
                        _Linear_Tree = _Linear_Tree.Insert(j + 1, "()");
                        break;
                    }
                }
            }
            _Linear_Tree = _Linear_Tree.Replace("e","");
            return _Linear_Tree;
        }

        static bool output_possible(string sequence, StringBuilder S)
        {
            int F = 0;
            for (int i = 0; i < S.Length; i++)
            {
                switch (S[i])
                {
                    case 'S':
                        if (F < sequence.Length && (sequence[F] == '1' || sequence[F] == '2' || sequence[F] == '3'))
                        {

                                S[i] = '0';
                                ReplaceChar(S, Convert.ToInt32(sequence[F]) - 48);
                                F++;
                                i = 0;
                                break;

                        }
                        else
                            return false;

                    case 'A':
                        if (F < sequence.Length && (sequence[F] == '4' || sequence[F] == '5' || sequence[F] == '6'))
                        {

                                S[i] = '0';
                                ReplaceChar(S, Convert.ToInt32(sequence[F]) - 48);
                                F++;
                                i = 0;
                                break;

                        }
                        else
                            return false;
                    case 'B':
                        if (F < sequence.Length && (sequence[F] == '7' || sequence[F] == '8'))
                        {

                                S[i] = '0';
                                ReplaceChar(S, Convert.ToInt32(sequence[F]) - 48);
                                F++;
                                i = 0;
                                break;

                        }
                        else
                            return false;
                }
            }
            return true;
        }

        static void Main(string[] args)
        {
            Console.WriteLine("Enter 1 for manual mode\nEnter 2 for sequence mode");
            var S = new StringBuilder("S");
            char mode = Console.ReadKey().KeyChar;
            if (mode == '1')
            {
                var sequence = new StringBuilder("");
                Console.WriteLine("\nChoose Left (l) or Right (r): ");
                char direction = Console.ReadKey().KeyChar;
                switch (direction)
                {
                    case 'l':
                        Console.WriteLine("\nLeft output");
                        S = grammar_outL(S, sequence);
                        break;
                    case 'r':
                        Console.WriteLine("\nRight output");
                        S = grammar_outR(S, sequence);
                        break;
                    default:
                        Console.WriteLine("\nerror");
                        break;
                }
                Console.WriteLine("Final word: " + S);
                Console.Write("Sequence: ");
                for (int i = 0; i < sequence.Length; i++)
                {
                    Console.Write(sequence[i] + " ");
                }
                Console.WriteLine("\n\n" + Linear_Tree(sequence));
            }
            else if (mode == '2')
            {
                Console.WriteLine("\nInput sequence: ");
                string sequence;
                sequence = Console.ReadLine();
                switch(output_possible(sequence, S))
                {
                    case true: Console.WriteLine("Yes"); break;
                    case false: Console.WriteLine("No"); break;
                }
            }
            else
                Console.WriteLine("error");

            Console.WriteLine("\nPress any key to exit");
            Console.ReadLine();
        }
    }
}