Assignment 5

 import java.io.*;
 import static java.lang.System.*;
 import java.util.Random;
 import java.util.Scanner;
 import java.lang.Math;

 class Main{


      public static void main (String str[]) throws IOException {
        Scanner scan = new Scanner(System.in);
        System.out.println("Enter the first String:");
        String fst = scan.nextLine();
        System.out.println("Enter the replacement String:");
        String snd = scan.nextLine();
        int flag = 0;
        CharSequence s1 = "%";
        CharSequence s2 = snd.substring(0);

        for(int i = 0; i < fst.length(); i ++)
        {
        if (fst.charAt(i) == '%')
            {
              if (fst.matches("^.*[^a-zA-Z% ].*$") && flag == 0)
              {
                System.out.println("Error: Incorrect characters");
                flag += 99;
              }
              flag++;
            }
        }
         if (flag == 1)
              {
                System.out.print(fst.replace(s1 , s2));
              }
         else if (flag > 1 && flag < 98)
           {
                System.out.println("Error: Incorrect characters");
         }
        if (flag == 0)
          System.out.println("Error: no %");
      }
 }