Add new bbcode and extract the text

[drdaco]

I want to use this cgi http://www.forkosh.com/mimetex.html

like this:

[tex] x=\frac{-b\pm\sqrt{b^2-4ac}}{2a}[/tex]

where the text in the middle can be parsed like

 <img src="/cgi-bin/mimetex.cgi?
    x=\frac{-b\pm\sqrt{b^2-4ac}}{2a}">

once the user hit's "post message"

so how do I make a new bbcode do that?

thanks for the help