$re = '/<div\sclass="portal"(.*?)<\/div>/s';
$str = '<div class="portal" role="navigation" id=\'p-participation\' aria-labelledby=\'p-participation-label\'>
<h3 id=\'p-participation-label\'>Раз раз раз</h3>
</div> <div class="portal" role="navigation" id=\'p-tb\' aria-labelledby=\'p-tb-label\'>
<h3 id=\'p-tb-label\'>Инструменты</h3>
</div> <div class="portal" role="navigation" id=\'p-coll-print_export\' aria-labelledby=\'p-coll-print_export-label\'>
<h3 id=\'p-coll-print_export-label\'>Печать/экспорт</h3>
</div> <div class="portal" role="navigation" id=\'p-wikibase-otherprojects\' aria-labelledby=\'p-wikibase-otherprojects-label\'>
<h3 id=\'p-wikibase-otherprojects-label\'>В других других</h3>
</div> <div class="portal" role="navigation" id=\'p-lang\' aria-labelledby=\'p-lang-label\'>
<h3 id=\'p-lang-label\'>Три три три</h3>
</div>
';
preg_match_all($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