import java.util.*;
import java.util.regex.Pattern;

public class EvaluateDemo {
	public static final Pattern CHARACTER = Pattern.compile("\\S.*?");

	public static final Pattern INT = Pattern.compile("[+-]?\\d+.*?");

	public static final Pattern UNSIGNED_DOUBLE = Pattern.compile("((\\d+\\.?\\d*)|(\\.\\d+))([Ee][-+]?\\d+)?.*?");

	public static final Pattern UNSIGNED_INT = Pattern.compile("\\d+.*?");
	
	public static void main(String[] args) {
		
		String expr;

		Scanner input = new Scanner(System.in);

		System.out.print("Enter an expression: ");

		expr = input.nextLine();
		
		double x = evaluate(expr);
		
		System.out.println("Answer = " + x);
	}

	public static double evaluate(String expression) {
		
		Stack<Double> numbers = new Stack<Double>();
		Stack<Character> operations = new Stack<Character>();
		
		Scanner input = new Scanner(expression);
		String next;
		
		while (input.hasNext()) {
			if (input.hasNext(UNSIGNED_DOUBLE)) {
				next = input.findInLine(UNSIGNED_DOUBLE);
				numbers.push(new Double(next));
			} else {
				next = input.findInLine(CHARACTER);
				switch (next.charAt(0)) {
				case '+':
				case '-':
				case '*':
				case '/':
					operations.push(next.charAt(0));
					break;
				case ')':
					evaluateStackTops(numbers, operations);
				break;
				case '(':
					break;
					default:
						throw new IllegalArgumentException("Illegal character");
				}
			}
		}
		if (numbers.size() != 1 || !operations.isEmpty()) {
			throw new IllegalArgumentException("Illegal input expression");
		}
			
		return numbers.pop();
	}

	public static void evaluateStackTops(Stack<Double> numbers,
			Stack<Character> operations) {
		double operand1, operand2;

		if ((numbers.size() < 2) || (operations.isEmpty()))
			throw new IllegalArgumentException("Illegal expression");
		operand2 = numbers.pop();
		operand1 = numbers.pop();

		switch (operations.pop()) {
		case '+':
			numbers.push(operand1 + operand2);
			break;
		case '-':
			numbers.push(operand1 - operand2);
			break;
		case '*':
			numbers.push(operand1 * operand2);
			break;
		case '/':
			numbers.push(operand1 / operand2);
			break;
		default:
			throw new IllegalArgumentException("Illegal operation");
		}
	}
}