Java - how to validate following input format 'operand operator operand' (e.g. 1.4 * 5)

Question

I'm breaking my head trying to figure out what is wrong in my code. I'm asking the user to insert the following format:

LeftOperand operator RightOperand

Where:

• LeftOperand - can be any int or float number
• operator - can be +, -, * or /
• RightOperand - can be any int or float number

I searched and found few regular expressions but they all doesn't seems to work for me, as anything besides the following: 3 +/-/*// 5 returns an error. Should be valid:

• 1 + 3
• 1 / 4
• 1.3 - 4
• 4 - 5.3
• 123 * 3434
• 12.34 * 485

but in fact only those are valid while the rest return error:

• 1 + 3 - correct
• 1 / 4 - correct
• 1.3 - 4 - error
• 4 - 5.3 - error
• 123 * 3434 - error
• 12.34 * 485 - error

I'm currently using the following regex in my code:

"[((\\d+\\.?\\d*)|(\\.\\d+))] [+,\\-,*,/] [((\\d+\\.?\\d*)|(\\.\\d+))]"

Tried all kind of regex, but none seems to work, and I just don't know what am I doing wrong:

[[+-]?([0-9]*[.])?[0-9]] [+,\-,*,/] [[+-]?([0-9]*[.])?[0-9]]

This is my client code:

private static final int SERVER_PORT = 8080;
private static final String SERVER_IP = "localhost";
private static final Scanner sc = new Scanner(System.in);
private static final String regex = "[((\\d+\\.?\\d*)|(\\.\\d+))] [+,\\-,*,/] [((\\d+\\.?\\d*)|(\\.\\d+))]";

public static void main(String[] args) throws IOException {
    // Step 1: Open the socket connection
    Socket serverSocket = new Socket(SERVER_IP, SERVER_PORT);

    // Step 2: Communication-get the input and output stream
    DataInputStream dis = new DataInputStream(serverSocket.getInputStream());
    DataOutputStream dos = new DataOutputStream(serverSocket.getOutputStream());

    //Run until Client decide to disconnect using the exit phrase
    while (true) {
        // Step 3: Enter the equation in the form -
        // "operand1 operation operand2"
        System.out.print("Enter the equation in the form: ");
        System.out.println("'operand operator operand'");

        String input = sc.nextLine();
        // Step 4: Checking the condition to stop the client
        if (input.equals("exit"))
            break;

        //Step 4: Check the validity of the input and
        // return error message when needed
        if (input.matches(regex)) {
            System.out.println("Expression is correct: " + input);
        } else {
            System.out.println("Expression is incorrect, please try again: ");
            continue;
        }

        // Step 5: send the equation to server
        dos.writeUTF(input);

        // Step 6: wait till request is processed and sent back to client
        String ans = dis.readUTF();
        // Step 7: print the response to the console
        System.out.println("Answer=" + Double.parseDouble(ans));
    }
}

Answer 1

private static final String NUM = "(-?\\d+(\\.\\d*)?)";
private static final String OP = "([-+*/])"; // Minus at begin to avoid confusion with range like a-z.
private static final String WHITESPACE = "\\s*";
private static final String regex = NUM + WHITESPACE + OP + WHITESPACE + NUM;

Note:

• $1 corresponds with the first number
• $3 corresponds with the operator
• $4 corresponds with the second number
• (...) is a group ($1, $2, ...; $0 is all)
• [...] is a character set, one char match
• X? X optional
• X* X 0 or more times
• X+ X 1 or more times
• \\s space, tab
• \\d digit