<r:if_url matches="^.\b(podcast|podcasts|audio|media)\b.$">
some reference to an audio file here
</r:if_url>
Essentially what the regular expression above does is show the content
only if the url contains any of the following words. So in the example
if I’m running a podcast (using page_attachments to upload audio files)
I can show the audio url ONLY within the context of the url keywords
described in the regexp and you can add as many keywords as you’d like.