$re = '/<textarea[^>]+data-match-this="[^"]+"[^>]*>.*?<\/textarea>/ms';
$str = '<textarea data-match-this="true">{{one}}{{two}}</textarea>
<textarea>{{three}}{{four}}</textarea>
<textarea data-match-this="true">
{{five}}
{{six}}{{seven}}
</textarea>
<textarea>
{{eight}}
{{nine}}{{ten}}
</textarea>';
preg_match_all($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