How to find a unique string within html and wrap it with a tag, but exclude links and urls

I’m looking for a way to look for a specific string within a page and then wrap that string in <em> tags. I have tried used HTML Agility Pack and had some success with a Regex.Replace but if the string is included within a url it also gets replaced, if it’s within an image name, it gets replaced and this obviously breaks the link or image url.

An example:

   var markup = Encoding.UTF8.GetString(buffer);
        var replaced = Regex.Replace(markup, "product-xs", " <em>product</em>-xs", RegexOptions.IgnoreCase);
        
        var output = Encoding.UTF8.GetBytes(replaced);
    
        _stream.Write(output, 0, output.Length);

This would replace a <a href="product/product-xs"> with <a href="product/<em>product</em>-xe">

The string is coming from a text string value within a CMS so the user can’t wrap the words there and ideally, I want to catch all instances of the word that are already published.

Ideally I would want to exclude <title> tags, <img> tags and <a href> and <a> tags, everything else should get the wrapped tag.

Before I used the HTML Agility Pack, a fellow front end dev tried it with javascript but that had an unexpected impact on dropdown menus.

If you need any more info, just ask.