"Why GNU grep is fast:"
http://lists.freebsd.org/pipermail/freebsd-current/2010-August/019310.html

(in the thread, a link of interest: http://swtch.com/~rsc/regexp/)
 also: http://lists.freebsd.org/pipermail/freebsd-current/2010-August/019352.html
       http://lists.freebsd.org/pipermail/freebsd-current/2010-August/019357.html
       etc.