$re = '/(?:width|height):\s?\d+px;?\s?/';
$str = '<div id="attachment_9565" class="wp-caption aligncenter" style="width: 2010px;background-color:red;height:200px">';
$subst = "";
$result = preg_replace($re, $subst, $str);
echo "The result of the substitution is ".$result;
Please keep in mind that these code samples are automatically generated and are not guaranteed to work. If you find any syntax errors, feel free to submit a bug report. For a full regex reference for PHP, please visit: http://php.net/manual/en/ref.pcre.php