$re = '/-\s+\[[^:]+:(?<first_name>[^_]+)_(?<last_name>\w+)@[^]]+\] - User authenticated$/';
$str = '2014-10-10 09:58:27 EDT (Framework:INFO) [RID:526] - [sthisisencryptedpassword:firstname_lastname@
serial.mobile.com] - User authenticated';
preg_match($re, $str, $matches, PREG_OFFSET_CAPTURE, 0);
// Print the entire match result
var_dump($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