[Mimedefang] Regexp help

[Ashley M. Kirchner]

    I seem to be getting a lot of spam from e-mails that start with 
either a '-' or '_', for example 'from=<-l-i-s-h-a at adam-friedman.com>' 
and 'from=<_nia at adidassler.com>'.  Obviously bouncing them is a waste of 
time.  Can someone help with a regexp that I can stick in my config to 
tarpit these guys (/dev/null) ?  Or, if there's a better solution, I'm 
listening.

    Running Sendmail/MD/SA