$re = '~<script>((?!</script).)*?site(?1)*</script>~si';
$str = '<script>
o.com
bla bla bla
</script><p>this is line></p>
<script>
google.com/jquery.js !
</script>
<script>
site.com
bla bla bla
</script><p>aa</p>
CONTENT
STYLE
SIDEBAR
...
<script>
site.com
aaa bla bla bla
</script><p>a</p>';
preg_match_all($re, $str, $matches, PREG_SET_ORDER, 0);
// Print the entire match result
var_dump($matches);
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