$re = '/<div>((?:<div[^>]*>(?1)<\/div>|[^<]*|<(?!div))*)<\/div>/msi';
$str = '<div>
Lorem ipsum dolor sit amet
</div>
<div>
Lorem ipsum dolor sit amet
</div>
<div>
<div class="media_embed">
<div style="xxx">
<div style="xxx">
<div style="xxx"></div>
</div>
</div>
</div>
</div>
<div>
Lorem ipsum dolor sit amet
</div>
<div class="foo">
<div>FOO</div>
<div class="bar">
<div>BAR</div>
</div>
<div>FOOBAR<div class="foobar"></div></div>
<div class="bar"><div><b>BAR</b></div></div>
<!-- para este caso, deberás ejecutar preg_replace
hasta que no haya nada nuevo por reemplazar -->
<div><div>foobar</div></div>
</div>';
$subst = "<p>$1</p>";
$result = preg_replace($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