Because this question is tagged with regex I'm going to answer with poor man's solution in this situation:
$html = preg_replace('##is', '', $html);
However, regular expressions are not for parsing HTML/XML, even if you write the perfect expression it will break eventually, it's not worth it, although, in some cases it's useful to quickly fix some markup, and as it is with quick fixes, forget about security. Use regex only on content/markup you trust.
Remember, anything that user inputs should be considered not safe.
Better solution here would be to use
DOMDocument which is designed for this.
Here is a snippet that demonstrate how easy, clean (compared to regex), (almost) reliable and (nearly) safe is to do the same:
$script = $dom->getElementsByTagName('script');
$remove = ;
foreach($script as $item)
$remove = $item;
foreach ($remove as $item)
$html = $dom->saveHTML();
I have removed the HTML intentionally because even this can bork.