Highlight all words from an HTML page

Suppose an HTML page like this:

 <h1>My page</h1>
 <div>This is an HTML page...</div>
 <div>Hello. Welcome to HTML, HTML5 is easy to learn</div>

I want to highlight all words individually, but not any special character or number.

I tried the following code, but my regular expression doesn't match:

if (node.nodeType === 3) { // Node.TEXT_NODE
    var text = node.data,
        pos = text.search(/any regular expression/g), //indexOf also applicable
        length = 5; // Or whatever you found
    if (pos > -1) {
        node.data = text.substr(0, pos); // Split into a part before...
        var rest = document.createTextNode(text.substr(pos+length)); // A part after
        var highlight = document.createElement("span"); // And a part between
        highlight.className = "highlight";
        highlight.appendChild(document.createTextNode(text.substr(pos, length)));
        node.parentNode.insertBefore(rest, node.nextSibling); // insert after
        node.parentNode.insertBefore(highlight, node.nextSibling);
    }
}

How do I write a regular expression that takes only words and not spaces or special characters?