The file looks something like this once I run my extension:

What it does is find every occurrence of a user-specified word and wraps it in an HTML "bold" tag. Then it styles the word with a color attribute.

The app I used made the process easier before I knew how to do regular expressions (learned regular expressions in a day). But with regular expressions I was able to do a more complex task, like comb a document for all emails, sort them alphabetically, and replace all text in the document with only the emails, in a list.