Regular Expression to verify condition in JavaScript

Question

I am a new to JavaScript. I have tried to use regular expression in ifcondition. Here is my code:

var location1 = 3;
var location2 = 4;
var location3 = 5;

var guess;
var hits = 0;
var guesses = 0;
var string = "";
var reg = /[0-6]/i;

var isSunk = false;

while (isSunk == false) {
guess = prompt("Ready, aim, fire! (enter a number 0-6): ");
if (guess != reg || guess ==string) {
    alert("Please enter a valid cell number!");
} else {
    guesses = guesses + 1;

if (guess == location1 || guess == location2 || guess == location3) {

        hits = hits + 1;
        alert("HIT!");

        if (hits == 3) {
            isSunk = true;
            alert("You sank my battleship!");
        }
    } else {
        alert("MISS!");
    }
}
}
var stats = "You took " + guesses + " guesses to sink the battleship, " +    "which means your shooting accuracy was " + (3 / guesses);
alert(stats);`

My problem is in this if (guess != reg || guess ==string) condition. It didn't work as I expected. I would like that function prompt allows only 0,1,2,3,4,5,6 numbers without any words and spaces. But in fact If I enter one of needed number (1 for example) it won't allow to execute further actions. I read a lot through stackoverflow about regular expression in Javascript (Convert the Regular expression, Regular expression, Basic regular expression) and more but I could not find the answer to my problem. So the question is:

How to set proper regular expression in if condition in JavaScript if it has already declared as variable?

You need to test your regular expression against its value, w3schools.com/jsref/jsref_regexp_test.asp — Murtaza Khursheed Hussain
– Murtaza Khursheed Hussain, Commented Feb 29, 2016 at 13:04
guess != reg here you compare a user string with a regex object; this is not correct, you need to reg.test(guess) You also need to anchor the regex /^[0-6]$/ to make it match 1 digit — Alex K.
– Alex K., Commented Feb 29, 2016 at 13:05
Murtaza, actually, I have already done this here regexr.com and everything was alright — fuser
– fuser, Commented Feb 29, 2016 at 13:07

Julian Kniephoff · Accepted Answer · 2016-02-29 13:18:24Z

1

To test whether a string matches a regular expression you can't just compare them using the standard comparison operators. Something that should work is

if (reg.test(guess)) {
    ....
}

Also the regular expression [0-6]i also matches the string "hello1, since there is a one in it. What you want is probably something like ^[0-6]$. The test for the empty string is also unnecessary, by the way, since "" does not match that expression.

To explain: The ^ and the $ match the beginning and the end of the string, respectively. Framing a regular expression with these symbols essentially says that the whole string, not only a substring, has to match the expression.

answered Feb 29, 2016 at 13:18

Julian Kniephoff

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HBP · Accepted Answer · 2016-02-29 13:12:53Z

0

Regular expression are not the right tool here.

 guess = prompt("Ready, aim, fire! (enter a number 0-6): ");
 guess = +guess;   // Convert *guess* to number, sets to NaN if not numeric
 if (guess >= 0 && guess <= 6) {  // check if number is in range (NaN will fail)
     ....

answered Feb 29, 2016 at 13:12

HBP

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