Deciphering

package DemoFinalExam;

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;

public class Deciphering {
    public static void main(String[] args) throws IOException {
        BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));

        String line = reader.readLine();
        String newLine = "";

        if (isValid(line)) {

            for (int i = 0; i < line.length(); i++) {
                char symbol = (char) (line.charAt(i) - 3);
                newLine += symbol;

            }

            String substring = reader.readLine();
            String[] separateSubstrings = substring.split(" ");
            String firstSub = separateSubstrings[0];
            String secondSub = separateSubstrings[1];

            String str = newLine.replaceAll(firstSub, secondSub);


            System.out.println(str);
        } else {
            System.out.println("This is not the book you are looking for.");
        }
    }

    private static boolean isValid(String firstLine) {
        for (int i = 0; i < firstLine.length(); i++) {
            int symbol = (int) (firstLine.charAt(i));
            if (!(symbol == 35 || (symbol >= 100 && symbol <= 125))) {
                return false;
            }
        }
        return true;
    }
}