ExpressionParser.java

package appl_alg_interpreter;

import java.io.File;

public class ExpressionParser {

	static ProgramScanner psc;

	static String NAME = "[a-zA-Z_][a-zA-Z_0-9]*";
	static String NUMBER = "(^|\\s+)((\\-?[0-9]+(\\.[0-9]*)?)|(-?\\.[0-9]+))($|\\s+)";
	static String OPERATOR = "[\\+\\-\\*/]";

	static class Node {
		public String value;
		public Node left;
		public Node right;

		public Node(String value, Node left, Node right) {
			this.value = value;
			this.left = left;
			this.right = right;
		}

		public static Node expr(ProgramScanner psc) throws Exception {
			String token = psc.nextTokenOrNull(NUMBER);
			if (token != null) {
				return new Node(token, null, null);
			}
			token = psc.nextTokenOrNull("\\(");
			Node left = Node.expr(psc);
			token = psc.nextToken(OPERATOR);
			String value = token;
			Node right = Node.expr(psc);
			token = psc.nextTokenOrNull("\\)");

			return new Node(value, left, right);
		}
	}

	public static void main(String[] args) {

		try {
			psc = new ProgramScanner(new File("appls/appl_algs/Test.apl"));
			Node result = Node.expr(psc);

			System.out.println("Success");
			System.out.println(result.left.value);
			System.out.println(result.right.value);
			System.out.println(result.value);
		} catch (Exception e) {
			System.out.println(e.getMessage());
		}
	}

}

Well done :)