How to modify post content before output to blog

Hi all!

I am setting up a blog where most/all of the post are excpected to contain an identical set of information. Since I would like to structure these pieces of information a bit in the output of each post – and alwas in identical fashion, I intend to write a plugin that does just that.

Basically, it should recognize a couple of tags that I would use in all of my post, e.g.,

[item1]MyContent[/item1]
[item2]MyContent[/item2]
...
etc.

so as for the plugin to be able to parse the post’s content and replace the tags with HTML-elements that would allow me to build the desired structure and add the desired formatting (via CSS).

I have gotten as far as identifyfing the relevant hook

add_filter('the_content', 'myFunction');

and linking my stylesheet

echo '<link type="text/css" rel="stylesheet" href="' . get_bloginfo('wpurl') . '/wp-content/plugins/myPath/myStyleSheet.css" />' . "\n";.

I am suspecting that I will have to retrieve

$post->post_content

to apply my modifications, but this is just a suspicion, since I have not been able to test anything yet.

I am failing, however, to understand how I return my modified content (which I expect to be stored in a string variable after the above operations) to ‘the loop’ so that it will be output to screen.

Can anyone give me the necessary spark of inspiration?

TIA

Matthias