How do I add a media provider?

I regularly post articles in my forum for a tech blog that I write for davescomputertips.com and would like to see them come up as a preview.
I see that I can add a provider, but I don't know the regular expression or the html code required to do this.
Any ideas?
Many thanks.

I don't really understand your answer if the Media Providers plugin that you provide allows a preview like this:

Why then can't you provide the necessary code for another media provider? Or, to put it another way, why have '+add media provider' in your plugin?