The following is not one that I wrote, but it is the first example of a complex regular expression that I ever encountered - so I always remember it. It can be found in the procmailrc(5) manpage.

 (^(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 ][^<)]*(\(.*\).*
+)?)?$([^>]|$)))
[download]

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.

Cheers,
Darren :)