$re = '/<div class="whatever">(.*?)(<div[^>]*>((?>(?:(?!<\/?div)[^\n])+|\n|(?2))*)<\/div>)(.*?)<\/div>/mus';
$str = '
<div class="tutu">
Blahblah.
<div class="whatever">
Blahblah0.
<div class="tata">
Blahblah1.
</div>
Blahblah2.
</div>
Blahblah3.
</div>
';
$subst = "\1\2\4";
$result = preg_replace($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