Moving characters ASCII up and down depending if the input is valid

No such

using System;
using System.Linq;
using System.Text;
using System.Text.RegularExpressions;

namespace ConcertRegex
{
    class Program
    {
        static void Main(string[] args)
        {
            string artistPattern = @"^(?<artist>[ A-z']*)$";
            string songPattern = @"^(?<song>[A-Z ]*)$";
            string keepPattern = @"[^ @']";
            string line = string.Empty;

            while ((line = Console.ReadLine()) != "end")
            {
                string[] tokens = line.Split(":");
                string artist = tokens[0];
                string song = tokens[1];

                bool artistIsValid = Regex.IsMatch(artist, artistPattern);
                bool songIsValid = Regex.IsMatch(song, songPattern);

                if (!artistIsValid ||!songIsValid)
                {
                    Console.WriteLine("Invalid input!");
                }
                else if (artistIsValid && songIsValid)
                {
                    Match artistMatch = Regex.Match(artist, artistPattern);
                    Match songMatch = Regex.Match(song, songPattern);

                    StringBuilder sb = new StringBuilder();
                    int length = artist.Length;
                    string text = $"{artistMatch.Groups["artist"].Value}@{songMatch.Groups["song"].Value}"; // The wanted format

                    foreach (char symbol in text)
                    {
                        char newSymbol = symbol;
                        bool isValidSymbol = Regex.IsMatch(newSymbol.ToString(), keepPattern); // Check if this is valid

                        if (isValidSymbol)
                        {
                            newSymbol += (char)length; // The number of this result will be converted to char and moved up by the length

                            if (symbol <= 90 && newSymbol >= 90) // If its less or bigger than Z
                            {
                                newSymbol -= (char)26; // Change it to A
                            }
                            else if (symbol <= 122 && newSymbol >= 122) // If its less or bigger than z
                            {
                                newSymbol -= (char)26; // Change it to a
                            }
                        }
                        //sb += newSymbol.ToString(); cannot do this opperation, therefor append can be used
                        sb.Append(newSymbol);
                    }
                    Console.WriteLine($"Successful encryption: {sb}");
                }
            }
        }
    }
}