skip WHITESPACE '\s+'

token OP '\+|-'
token MOP '\*|/'
token NUM '[0-9]+'

%

<start> ::= <exp>
<exp> ::= <mult> <exprest>
<exprest> **= <OP> <mult>
<mult> ::= <value> <multrest>
<multrest> **= <MOP> <value>
<value> ::= <NUM>

%

Start
%%{

   public void $run() {
      System.out.println(exp.calculate());
   }

%%}

Exp
%%{

   public int calculate() {
      int x = mult.calculate();
      return exprest.calculate(x);
   }

%%}

Exprest
%%{

   public int calculate(int resultSoFar) {
      Iterator<Token> opIt = opList.iterator();
      Iterator<Mult> multIt = multList.iterator();

      while(opIt.hasNext()) {
         String op = opIt.next().toString();
	 int n = multIt.next().calculate();

         switch(op) {
	    case "+": resultSoFar += n; break;
	    case "-": resultSoFar -= n; break;
	 }

      }

      return resultSoFar;
   }

%%}

Mult
%%{

   public int calculate() {
      int x = value.getValue();
      return multrest.calculate(x);
   }

%%}

Multrest
%%{

   public int calculate(int resultSoFar) {
      Iterator<Token> mopIt = mopList.iterator();
      Iterator<Value> valueIt = valueList.iterator();

      while(mopIt.hasNext()) {
         String op = mopIt.next().toString();
	 int n = valueIt.next().getValue();

         switch(op) {
	    case "*": resultSoFar *= n; break;
	    case "/": resultSoFar /= n; break;
	 }

      }

      return resultSoFar;
   }

%%}

Value
%%{

   public int getValue() {
      return Integer.parseInt(num.toString());
   }

%%}