There probably is a bit short way to do this, but here is how it could be done. As for the actual regex, you might want to take a look at the markup functions if you can't get that part to work.
// Declaring an array before hand
$piece[0] = "";
// Grab the bit you are looking for
preg_match("/pattern_to_look_for/", $stringtosearch, $piece);
// Call your function passing the bit stored in the array.
highlight_strng($piece[1]);