$re = '/<metaData\s+key="([^"]+)">((?!<\/metaData>).+)<\/metaData>/m';
$str = ' <metaDataSet>
<metaData key="DocName">mm1266505-20180330195251.txt</metaData>
<metaData key="TARGET">Brazil BES</metaData>
<metaData key="com.ibm.wmqfte.OriginatingUser">eimb@CORPWEB</metaData>
<metaData key="com.ibm.wmqfte.OriginatingHost">B051S317.jci.com</metaData>
<metaData key="com.ibm.wmqfte.TransferId">414d512050445053514d513031202020770fbe5aacf88f20</metaData>
<metaData key="com.ibm.wmqfte.Priority">0</metaData>
</metaDataSet>
';
preg_match_all($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