$re = '/<item>[\s\S]*?<sub>(.*?)<\/sub>((?!<sub>)[\s\S])*<\/item>/';
$str = '<item> fffffffffffff
<sub>aaaa</sub>
<value>111</value>
</item>
<item>
<sub>eeee</sub> arg34ddddddddddddddd
<atag>ddd</atag>
<sub>aaaa</sub>
<value>222</value>
</item>
<item>
<sub>eeee</sub> arg34ddddddddddddddd
<atag>ddd</atag>
<sub>aaaa</sub>
</item>';
preg_match_all($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