$re = '/<.[^>]*>(\s+|()|( )*|\s+( )*|( )*\s+|\s+( )*\s+)<\/.[^>]*>/i';
$str = '<p></p>
<p> </p>
<b>
</b>
<B></B>
<i> </i>
<i> </i>
<i> </i>
<strong></strong>
<i style="color: orange"></i>
<a href="#"> </a>
<o: class="ms-crap"></o>
<p>Not empty</p>
<p>Not
empty</p>
<i>
</i>
';
preg_match_all($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