(DEMO)Final Exam - 06 April 2019 - 02. Deciphering

package com.company;

import java.util.*;
import java.util.regex.Matcher;
import java.util.regex.Pattern;
import java.util.stream.Collectors;

public class Main {

    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);

        String encrypted = scanner.nextLine();
        String afterEncryption = "";
        List<String> letters = Arrays
                .stream(scanner.nextLine()
                .split(" "))
                .collect(Collectors.toList());

        String regex = "^([d-z\\{\\}\\|#]+)";
        Pattern pattern = Pattern.compile(regex);
        Matcher matcher = pattern.matcher(encrypted);

        System.out.println();

        if (!matcher.find()){
            System.out.println("This is not the book you are looking for.");
        }else {
            for (int i = 0; i < encrypted.length(); i++) {
                afterEncryption += (char)((int)encrypted.charAt(i) - 3);
            }
            String first = letters.get(0);
            String second = letters.get(1);
            if (afterEncryption.contains(first)) {
                int index = afterEncryption.indexOf(first);
                while (index != -1) {
                    afterEncryption = afterEncryption.replace(first, second);
                    index = afterEncryption.indexOf(first);
                }
            }


        }

        System.out.println(String.join(" ",afterEncryption));

    }
}