$re = '/<([^>\s]+)[^>]*>(?:\s*(?:<br \/>|<br>| | | | | | | )\s*|\s*)*<\/\1>/i';
$str = '<div class="redactor-styles redactor-in redactor-in-0" dir="ltr" aria-labelledby="redactor-voice-0" role="presentation" style="min-height: 150px;" contenteditable="true"><p><br></p><p></p>
<p><strong>Ячейка1</strong></p><p></p>
<p></p>
<p><strong>Ячейка2</strong></p><p></p>
<p></p>
<p><strong>Ячейка3</strong></p><p></p>
<p></p>
<p><strong>Ячейка4</strong></p><p></p>
<p></p>
<p><strong>Ячейка5</strong></p><p></p>
<p></p>
<p><strong>Ячейка 6</strong></p><p></p>
<p></p>
<p><strong>Ячейка7</strong></p><p></p>
<p></p>
<p><strong>Ячейка8</strong></p><p></p>
<p></p>
<p><strong>Ячейка8</strong></p><p></p>
<p></p>
<p><strong>Ячейка10</strong></p><p></p></div>';
preg_match_all($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