How to add a custom "tag" to markdown

Hi list,

In a project I need to add some kind of custom “tag” to markdown which
will be pre-processed by the application. In an otherwise normal bit of
markdown text (a news posting), I would like to be able to refer to
other posts, whose URL and description, etc, will be looked up by the
app and added. It might look something like this:

article:12345

the application would see that text in the post, look up what article
12345 is, and replace the reference with a link as appropriate.

What would be the best way to achieve this? Some sort of preprocessing?
Or is there an easy/elegant way to extend Markdown to support this kind
of thing “natively”?

thanks for any suggestions,

James