Span regex replacement

I have a text in my databse. For example:

Dummy Text Here...
<span class="youtube">nmkW544sK9U</span>

Dummy Text Here...
<span class="youtube">yUBKZvq5G2g</span>

...and I need it to be replaced with:

Dummy Text Here...
<iframe width="640" height="395" frameborder="0" allowfullscreen="" src="http://www.youtube.com/embed/nmkW544sK9U?rel=0"></iframe>

Dummy Text Here...
<iframe width="640" height="395" frameborder="0" allowfullscreen="" src="http://www.yo开发者_开发百科utube.com/embed/yUBKZvq5G2g?rel=0"></iframe> 

But I don't know regular expressions well enough and ask you to help me.

As a general rule, don't use regex to parse HTML. It will cause you pain.

The nicest way to do this is with a genuine DOM parser. PHP's DOMDocument is ideal.

For instance:

$dom = new DOMDocument;
$dom->loadHTML($yourHTML);

$xpath = new DOMXPath($dom);

$nodes = $xpath->query('//span[@class="youtube"]');

while ($node = $nodes->item(0)) {
    $iframe = $dom->createElement('iframe');
    $iframe->setAttribute('width', 640);
    $iframe->setAttribute('height', 395);
    $iframe->setAttribute('frameborder', 0);
    $iframe->setAttribute('allowfullscreen', '');
    $iframe->setAttribute('src', 'http://www.youtube.com/embed/' . $node->nodeValue . '?rel=0');

    $node->parentNode->replaceChild($iframe, $node);
}

$yourHTML = $dom->saveHTML();

Something along these lines should work.

$replacement = '<iframe width="640" height="395" frameborder="0" allowfullscreen="" src="http://www.youtube.com/embed/$1?rel=0"></iframe>';
preg_replace('/<span class="youtube">(\w+)<\/span>/', $replacement, $string);