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

November 27, 2021

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
etc.
but in fact only those are valid while the rest return erorr:
1 + 3 – correct
1 / 4 – correct
1.3 – 4 – return error
4 – 5.3 – return error
123 * 3434 – return error
12.34 * 485 – return error

I’m currently using the following regex in my code:
private static final String regex = "[((\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:

package com.company;

//import sun.security.x509.OtherName;

import java.io.DataInputStream;
import java.io.DataOutputStream;
import java.io.IOException;
//import java.net.InetAddress;
import java.net.Socket;
//import java.net.UnknownHostException;
import java.util.Scanner;
//import java.util.StringTokenizer;

public class Client {

    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));
        }
    }
}

>Solution :

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: