need a simple way to highlight a literal string

I have an app that uses the html response language template and I love the highlighting. I want to maintain all the existing highlighting but add a custom highlighting style and apply it to a regex match (actually just match on a literal string) in the document. Is there a c# code sample on your site that shows me how to do this?

I don't need all the advanced lexical state analysis. I just need to look for a string literal and highlight it with a specific foreground and background color without giving up the existing highlighting.