Javascript replace function error

Are you seriously using regex to process HTML when you have a DOM parser at your fingertips?

var content = document.getElementById('content');

function findTextNodes(root,ret) {
    // recursively descend into child nodes and return an array of text nodes
    var children = root.childNodes, l = children.length, i;
    ret = ret || [];
    for( i=0; i<l; i++) {
        if( children[i].nodeType == 1) { // ElementNode
            // excluding A tags here, you might also want to exclude BUTTON tags
            if( children[i].nodeName != "A") {
                findTextNodes(children[i],ret);
            }
        }
        if( children[i].nodeType == 3) { // TextNode
            ret.push(children[i]);
        }
    }
    return ret;
}
var textNodes = findTextNodes(content);

// now search those text node contents for matching tags.
var tags = ['zazie','johnny'], tagcount = tags.length, regexes, tag;
for( tag=0; tag<tagcount; tag++) {
    regexes[tag] = new RegExp("\b"+tags[tag]+"\b","i");
}

var node, match, index, tagtext, newnode;
while(node = textNodes.shift()) {
    for( tag=0; tag<tagcount; tag++) {
        if( match = node.nodeValue.match(regexes[tag])) {
            index = match.index;
            textNodes.unshift(node.splitText(index + tags[tag].length));
            tagtext = node.splitText(index);
            newnode = document.createElement('a');
            newnode.href = "";
            newnode.className = "esk-seo-plu-link";
            newnode.style.cssText = "background:red;color:white";
            tagtext.parentNode.replaceChild(newnode,tagtext);
            newnode.appendChild(tagtext);
        }
    }
}

// and done - no more action needed since it was in-place.

See it in action