I am looking for suitable replacement code that allows me replace the content inside of any HTML tag that has a certain class e.g.

$class = "blah";
$content = "new content";
$html = '<div class="blah">hello world</div>';

// code to replace, $html now looks like:
// <div class="blah">new content</div>

Bare in mind that:

1. It wont necessarily be a div, it could be <h2 class="blah">
2. The class can have more than one class and still needs to be replaced e.g. <div class="foo blah green">hello world</div>

I am thinking regular expressions should be able to do this, if not I am open to other suggestions such as using the DOM class (although I would rather avoid this if possible because it has to be PHP4 compatible).