$re = '/(dateAdded:\s\/Date\()\d+\)\/\}\]/m';
$str = 'MULTIVALUE FIELD "APPID" -first event apps: [ [-] { [-] addedById: 5d013c468 appId: 5d0d1fc13d418bdf5 dateAdded: /Date(1573503009489)/ MULTIVALUE FIELD APPID-last value which needs to be extracted addedById: 398 appId:ccaaadb dateAdded: /Date(1584128055615)/}]
';
preg_match_all($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