in reply to The craziest RegExes you ever created
(^(Mailing-List:|Precedence:.*(junk|bulk|list)|To: Multiple recipient +s of |(((Resent-)?(From|Sender)|X-Envelope-From):|>?From )([^>]*[^(. +%@a-z0-9])?(Post(ma?(st(e?r)?|n)|office)|(send)?Mail(er)?|daemon|m(md +f|ajordomo)|n?uucp |LIST(SERV|proc)|NETSERV|o(wner|ps)|r(e(quest|sponse)|oot)|b(ou +nce|bs\.smtp)|echo|mirror|s(erv(ices?|er)|mtp(error)?|ystem)|A(dmin(i +strator)?|MMGR |utoanswer))(([^).!:a-z0-9][-_a-z0-9]*)?[%@>\t ][^<)]*(\(.*\).* +)?)?$([^>]|$)))
I've never tried to dissect it, but looking at it now (and having a somewhat better understanding of regular expressions than I did when I first saw it) I guess it's really not all that complex at all - mainly just lots of alternation.