Now that we know how to match strings, we should also be able to replace substrings in a string.
The situation for this recipe is that one of your friends has subscribed to this aggregator that sends him blog posts in plain text. (I know—how great would that be for you, and so horrible for the maintainers of commercial websites!) The issue, though, is that the aggregator sends her only one file with all the content loaded into it. You can see the file and understand that it is, in fact, Markdown with YAML, and agree to help her make the file pure Markdown so that it can be rendered into clean HTML.
The file for use in this recipe is available in this book's GitHub repository.