Compiler RD Parser Lab exam

Main.java

import java.util.*;  
import java.io.*;    
import java.lang.*;  


/*
 *
 * program -> prog block
 * block -> begin stmtlist end
 * stmtlist -> stmt morestmt
 * morestmt -> ; stmtlist | є
 * stmt -> assign | repeatstmt
 * assign -> var = expr
 * repeatstmt -> repeat stmt until testexpt
 * testexpr -> var <= expr
 * expr -> + expr expr
 *  | -expr expr
 *      | var
 *          |digit
 *          var -> u | v| w
 *          digit -> 0 | 1 |2
 */

// prog  begin end repeat until -> treated as terminals

class Main{
    static int index = -1;
    static String[] st;
    static boolean match(String s){
        return s.equals(st[index++]);
    }
    static boolean program(){
        int back = index;
        if(match("prog") && block())
            return true;
        index = back;
        return false;
    }
    static boolean block(){
        int back = index;
        if(match("begin") && stmtlist() && match("end"))
            return true;
        index = back;
        return false;
    }
    static boolean stmtlist(){
        int back = index;
        if(stmt() && morestmt())
            return true;
        index = back;
        return false;
    }
    static boolean morestmt(){
        int back=index;
        if(match(";") && stmtlist()){
            return true;
        }
        index = back;
        return true;
    }
    static boolean stmt(){
        int back = index;
        if(assign())
            return true;
        else{
            index = back;
            if(repeatstmt())
                return true;
        }
        index = back;
        return false;
    }
    static boolean assign(){
        int back = index;
        if(var() && match("=") && expr())
            return true;
        index = back;
        return false;
    }
    static boolean repeatstmt(){
        int back = index;
        if(match("repeat") && stmt() && match("until") && testexpr())
            return true;
        index = back;
        return false;
    }
    static boolean testexpr(){
        int back = index;
        if(var() && match("<=") && expr())
            return true;
        index = back;
        return false;
    }
    static boolean expr(){
        int back = index;
        if(match("+") && expr() && expr())
            return true;
        else{
            index = back;
            if(match("-") && expr() && expr())
                return true;
            else{
                index = back;
                if(var())
                    return true;
                else{
                    index = back;
                    if(digit())
                        return true;
                }
            }
        }
        index = back;
        return false;
    }
    static boolean var(){
        int back = index;
        if(match("u"))
            return true;
        else{
            index = back;
            if(match("v"))
                return true;
            else{
                index = back;
                if(match("w"))
                    return true;
            }
        }
        index = back;
        return false;
    }
    static boolean digit(){
        int back = index;
        if(match("0"))
            return true;
        else{
            index = back;
            if(match("1"))
                return true;
            else{
                index = back;
                if(match("2"))
                    return true;
            }
        }
        index = back;
        return false;
    }

    public static void main(String []args){   
		Scanner sc = new Scanner(System.in);  
		System.out.print("Enter a string : ");
		String inp = sc.nextLine();           
		st = inp.split(" ");                  
		index=0;                              
		if(program() && match("$")){                
			System.out.println("Valid");      
		}                                     
		else                                  
			System.out.println("Invalid");    
		}                                         
}