you can use just array method instead you don't need for regex for that, for example..

const values = cart_cookie.split(',');
values.foreach(value => { 
 if(value != product_id){
   values.push(`${product_id}:1`);
   setCookie('cart', values.join(','), 7);
 }
});

Yep, your regex is off :)

It stumped me for a second, but its clear when you check what your "regex" string actually evaluates to: /product_id:d+/. This is because the string you're passing in sees the \d as a literal d. To fix this, just throw another \ in there so the original \ is whats being escaped. Before, you were matching things like "apple:ddddddd".

Once you do that, your code DOES seem to work, but maybe just not like you expect it to? I've put your code into a function, since you would -- presumably -- be calling this every time you want to add an item to the cart, and added a console.log statement to show the end value of the "cookie."

// Just as a stand in for actual values
let cart_cookie = 'apples:2,oranges:3,bananas:1';

function addCartItem(product_id) {
    const reg = new RegExp(`${product_id}:\\d+`);
    console.log(reg)

    if (!reg.test(cart_cookie)){
        const values = cart_cookie.split(',');
        values.push(`${product_id}:1`);

        console.log('cart_cookie is now ', values.join(','));
        setCookie('cart', values.join(','), 7);
    }
    else {
        console.log('already in cart.')
    }
}


addCartItem('apples');
// already in cart.

addCartItem('kiwis');
// cart_cookie is now apples:2,oranges:3,bananas:1,kiwis:1

Its fixed! 🥳 But... I'm not quite sure what your product ids look like, but if they contain special characters (e.g. periods, question marks, etc.) it'll cause some issues with how your regex performs. I doubt you have things like question marks in it, but something like this illustrates my point:

let cart_cookie = '123.15:3,2.5.141:1';

/* ... */

addCartItem('1.3.15');
// already in cart.

I know, its a rather unlikely scenario -- and it might not even apply to you if you know your product ids won't contain anything tricky -- but if you're goal is to build an online shop, you'll probably want to cover all your bases. Even fixing that, this still has a potential issue, as this will only let you add a quantity of 1 to an item thats not already in the cart, with no ability to increment it after that. Not sure if thats what you're going for.

This veers off of the main question you posed, but a potentially better solution might be using the browser's localstorage (or sessionstorage) to keep track of the cart. This would allow you to use more familiar data structures to store this info, rather than a string that you have to pray you're parsing correctly. More info on that here: https://developer.mozilla.org/en-US/docs/Web/API/Window/localStorage