JSFiddle - React, Tailwind, and code Playground

HTML

<link rel="stylesheet" href="http://ajax.googleapis.com/ajax/libs/jqueryui/1.8.4/themes/ui-lightness/jquery-ui.css">

<script src="http://www.website.com/123.js" ></script>
<h1>123 is a great number!</h1>
Everyone likes 123.

JavaScript

var regex = /123/g,
    replacement = 'Pi';

function replaceText(i,el) {
    if (el.nodeType === 3) {
        if (regex.test(el.data)) {
            el.data = el.data.replace(regex, replacement);
        }
    } else {
        $(el).contents().each( replaceText );
    }
}

$('body').each( replaceText );