$re = '/(?:(?:(?:<([^ ]+)(?:.*)>)\[\[(?:<\/\1>))|(?:\[\[))(?:(?:(?:<([^ ]+)(?:.*)>)(.+?)(?:<\/\2>))|(.+?))(?:(?:(?:<([^ ]+)(?:.*)>)\]\](?:<\/\5>))|(?:\]\]))/m';
$str = '[[plain wikilinks]]
<e>[[</e>first tag]]
<e dsss>[[</e>meta on first tag]]
<e dsss>[[</e><div>first tag + meta + container</div>]]
<e dsss>[[</e><div>first tag + meta + container + third tag</div><div>]]</div>
<e dsss>[[</e>forgot a bracket
<e dsss>[[</e>bracket containers<e>]]</e>
all is well';
$subst = "$3$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