replacing a tag with a function

i’ve read the plugin api docs, plugin guidelines, php.net documentation on preg_replace etc, but i still can’t figure out how to do a very simple thing in a plugin:

i need to have a post include a special tag which will be replaced by the result of a simple function, for example:

This is an [specialtag:variable1,variable2] example.

i figured out (mostly) how to replace the tag using the_content filter with preg_replace. i think i’ll figure out how to extract the variables from it, too. but what is the syntax which allows replacing the entire tag with the result of a custom function?

i tried this:

$content = preg_replace_callback("/string/",hello_world(),$content);

but what this does is print the result of hello_world() at the top of the content, instead of replacing the string.

thanks for any help.