JSFiddle - React, Tailwind, and code Playground

HTML

<div id="outer"><div>www.foo.com</div>www.bar.com</div>

CSS

div div {
    color: red;
}

JavaScript

function replaceWwwInNodes(node) {
    if (node.nodeType === 3) {
        node.textContent = node.textContent.replace(/\bwww\..+?(?:\s|$)/, "[url=$&]");
    }
    else {
        Array.prototype.forEach.call(node.childNodes, function (elem) {
            replaceWwwInNodes(elem);
        });
    }
}

replaceWwwInNodes(document.getElementById('outer'));