As per http://www.zabbix.com/forum/showthread.php?p=99651. The problem is actually observed on version 1.8.1. Auto registration action has condition "Host name like", which is assumed to match hostnames. It does only substring check. I submit this as a bug, as I believe that regex is natural to expect there. In any case, substring matching makes this feature only 20% as useful as it could be. Documentation on this subject is lacking, it isn't mentioned anywhere.