const input = document.getElementById('input');

const box = document.getElementById('box');

const label = document.getElementById('label');

​

const re = /^[0-9]{0,4}$/;

​

input.addEventListener('input', validate);

​

function validate(e) {

    const value = e.target.value.trim();

    console.log(value);

    if(!value) {

        label.textContent = '';

        box.className = "form-group";

        return;

    }

    if(re.test(value)) {

        box.className = "form-group has-success";

        label.textContent = "Well, it's ok for now!";

    } else {

        box.className = "form-group has-warning";

        label.textContent = "Oh dude, totally weak!";

    }

}

​