HIGHLIGHT THE CODE

Hi, i write here beacuse i have a problem about the basic code presentaion of punBB.

When a user posts a code with the CODE tag, the standard behaviour of punBB is to put it into a pre.
I've downloaded Geshi, that permits to highlight the code.

new Geshi($string);

I want to capture the string of code users post and put it into the Geshi constructor.
How i can do this and in which file?

Thanks a lot.