$re = '/<div\b[^<]*>(?:[^<]+(?:<(?!\/?div\b)[^<]*)*|(?R))*<\/div>\s*/i';
$str = 'Text outside any div
<div id="my_id"> bla bla
<div>
bla bla bla
<div style="some style here">
lalalalala
</div>
</div>
I\'m trapped in a div!
</div>
more text outside divs
<div>more divs here!
<div id="justbeingannoying">radiohead rules</div>
</div>';
$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