<?php

$str = '(STUNNEL-SECURITY AND NOT (
    eventtype=sec-stunnel-client-* 
 OR eventtype=sec-stunnel-crl-* 
 OR eventtype=sec-stunnel-loaded-* 
 OR eventtype=sec-stunnel-loading-* 
 OR eventtype=sec-stunnel-no-* 
 OR eventtype=sec-stunnel-openssl-* 
 OR eventtype=sec-stunnel-sconnect-* 
 OR eventtype=sec-stunnel-server-* 
 OR eventtype=sec-stunnel-service-* 
 OR eventtype=sec-stunnel-session-* 
 OR eventtype=sec-stunnel-socket-* 
 OR eventtype=sec-stunnel-ssl-* 
 OR eventtype=sec-stunnel-stats-* 
 OR eventtype=sec-stunnel-threading-* 
 OR eventtype=sec-stunnel-timeout-* 
 OR eventtype=sec-stunnel-various-* 
) )';

preg_match_all("~eventtype=\K\S+~", $str, $matches);

print_r($matches[0]);