$re = '/<([^>\s]+)[^>]*>(?:\s*(?:<br \/>| | | | | | | )\s*)*<\/\1>/m';
$str = '<div>
<h1>This is a html structure.</h1>
<p>This is not empty.</p>
<p></p>
<p><br /></p>
<p> <br />  </p>
<p> </p>
<p> </p>
</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