KR

import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.util.Scanner;

import javax.swing.JFrame;
import javax.swing.JMenu;
import javax.swing.JMenuBar;
import javax.swing.JMenuItem;
import javax.swing.JOptionPane;
import javax.swing.JScrollPane;
import javax.swing.JTextArea;


public class Main extends JFrame {


    private static final long serialVersionUID = 4648172894076113183L;

    private void parseSource(String source) {
        Scanner scan = new Scanner(source);
        int r1 = 0, r2 = 0;
        while (scan.hasNextLine()) {
            String line = scan.nextLine().split(";")[0].trim();
            if (line.length() == 0) {
                continue;
            }
            try {
                String command = line.substring(0, 3).toLowerCase();
                switch (command) {
                case "add":
                case "sub":
                case "mov":
                    break;
                default:
                    JOptionPane.showMessageDialog(this, "Compilation error!", null, JOptionPane.ERROR_MESSAGE);
                    return;
                }
                String operands[] = line.substring(3).split(",");
                if (operands.length != 2) {
                    JOptionPane.showMessageDialog(this, "Compilation error!", null, JOptionPane.ERROR_MESSAGE);
                    return;
                }
                String op1 = operands[0].toLowerCase().trim();
                String op2 = operands[1].toLowerCase().trim();
                if (op1.equals("r1")) {
                    int c = 0;
                    if (op2.equals("r1")) {
                        c = r1;
                    } else if (op2.equals("r2")) {
                        c = r2;
                    } else {
                        try {
                            c = Integer.parseInt(op2);
                        } catch (Exception e) {
                            JOptionPane.showMessageDialog(this, "Compilation error!", null, JOptionPane.ERROR_MESSAGE);
                            return;
                        }
                    }
                    switch (command) {
                        case "add":
                            r1 += c;
                            break;
                        case "sub":
                            r1 -= c;
                            break;
                        case "mov":
                            r1 = c;
                    }
                } else if (op1.equals("r2")) {
                    int c = 0;
                    if (op2.equals("r1")) {
                        c = r1;
                    } else if (op2.equals("r2")) {
                        c = r2;
                    } else {
                        try {
                            c = Integer.parseInt(op2);
                        } catch (Exception e) {
                            JOptionPane.showMessageDialog(this, "Compilation error!", null, JOptionPane.ERROR_MESSAGE);
                            return;
                        }
                    }
                    switch (command) {
                        case "add":
                            r2 += c;
                            break;
                        case "sub":
                            r2 -= c;
                            break;
                        case "mov":
                            r2 = c;
                    }
                } else {
                    JOptionPane.showMessageDialog(this, "Compilation error!", null, JOptionPane.ERROR_MESSAGE);
                    return;
                }
            } catch (StringIndexOutOfBoundsException e) {
                JOptionPane.showMessageDialog(this, "Compilation error!", null, JOptionPane.ERROR_MESSAGE);
                return;
            }
        }
        scan.close();
        JOptionPane.showMessageDialog(this, "r1 = " + r1 + "\nr2 = " + r2);
    }

    private void initControls() {
        final JTextArea text = new JTextArea();
        getContentPane().add(new JScrollPane(text));
        JMenuBar menuBar = new JMenuBar();
        JMenu menu = new JMenu("File");
        JMenuItem miRun = new JMenuItem("Run");
        JMenuItem miExit = new JMenuItem("Exit");
        menu.add(miRun);
        menu.add(miExit);
        menuBar.add(menu);
        setJMenuBar(menuBar);

        miRun.addActionListener(new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent e) {
                parseSource(text.getText());
            }
        });

        miExit.addActionListener(new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent arg0) {
                System.exit(0);
            }
        });
    }

    private Main() {
        initControls();
        setDefaultCloseOperation(EXIT_ON_CLOSE);
        setSize(400, 400);
    }

    public static void main(String[] args) {
        new Main().setVisible(true);
    }

}