JSFiddle - React, Tailwind, and code Playground

HTML

<div>
    <textarea id="textarea" rows="4" cols="50" onKeyUp="validate(event)"></textarea>
</div>

JavaScript

function validate(e) {
    console.log(e)
    var invalidcharacters = /[^0-9]/gi
    var phn = document.getElementById('textarea');
    if (invalidcharacters.test(phn.value)) {
        //e.value = e.value.replace(invalidcharacters, "");
        console.log("REPLACING");
        newstring = phn.value.replace(invalidcharacters, "");
        phn.value = newstring
    }
}