replace a text between tags in php

Question

I have a php fine that output the html of a webpage.

The html is stored in a variable

$html = "<pre> something</pre> la la la <b> ohohoh </b> uuhhu <pre> ....";

Before doing an echo, I would like to add colors the C++ code between the pre tags.

I already have a function color($string) that does this, but I cannot do color($html) otherwise also the text outside pre tags will be parsed.

What I need to do is to scan the $html string and send to the color function only the part of the text between pre tags (there may be several pre /pre ) and replace it with the output of the function color().

how can i do this ?