How can I match valid sequence of operation using Regular Expression in Java

Question

I just started to learn about Regular Expression with Java. I have this project from hyperskill.com called "Smart Calculator" it is like the Python interpreter where you can input a arithmetic operation and when you press enter there is an output.

I would like to match if the input is valid like 2 + 2 - 3 and won't match if the input is 2 + 2 - .

I have these regular expression (\s*[0-9]+\s*[\+\-\*\/]+\s*[0-9]+\s*){1,}

But when I tried to test with some inputs...

[In]    1 + 1
[Out]   Matched

[In]    1 + 1 + 1    // Not matched
[Out]   Not Matched

[In]    1 + 12 + 1  //But this one matched
[Out]   Matched

Answer 1

[az] is regexpese for: Any character between a and z. So, your [+-*/] is regexpese for: "Any character having a unicode codepoint value between the codepoint value of + and the codepoint value of * which is problematic. Try putting the - at the start or end, in which case it does mean literally the - character, or escape it. So, make that: [-+*/] .

Secondly, your regexp is for:

• A number
• an operator
• A number

which you then state as: This, at least once or more (note that {1,} is just a silly way of writing + !

thus, you've set up the regexp to look for example for:

• A number
• an operator
• A number
• A number
• an operator
• A number

That duplication of 'a number' in the middle is your problem. The reason 1+12+1 matches is because your regexp matches that as 1+1 followed by 2+1 . That's also the reason why 1+1+1 does not match.

The solution is to start with a number, and then repeat only the 'operator, then number' part.

Note that \d is short for digit, shorter than [0-9].

Finally, you've said that any number of operators is okay, which makes no sense. try it: 1++1 is valid according to your regex here.

Putting it all together:

\s*\d+(\s*[-+*/]\s*\d+)+

is all you need.

Answer 2

Use the regex, (\s*[0-9]+\s*([+-]+\s*[0-9]+\s*)*) . Check this for an explanation of the regex.

public class Main {
    public static void main(String[] args) {
        String[] arr = { "1 + 1", "1 + 1 + 1", "2 + 2 - ", "2 + 2 - 3", "1 + 12 + 1" };
        for (String s : arr) {
            System.out.println(s + " => " + s.matches("(\\s*[0-9]+\\s*([+-]+\\s*[0-9]+\\s*)*)"));
        }
    }
}

Output:

1 + 1 => true
1 + 1 + 1 => true
2 + 2 -  => false
2 + 2 - 3 => true
1 + 12 + 1 => true