the shorter the regexp, the longer the grep time

[Dan Jacobson]

Gentlemen, what's the deal in that the shorter the regexp, the longer
the grep time?
# time zcat /var/log/syslog.*.gz|grep . > /dev/null
real    0m28.991s
user    0m28.990s
sys     0m0.000s
# time zcat /var/log/syslog.*.gz|grep fetchmail > /dev/null
real    0m6.556s
user    0m6.550s
sys     0m0.000s
# time zcat /var/log/syslog.*.gz|grep a > /dev/null
real    0m28.200s
user    0m28.190s
sys     0m0.010s
# grep --version
grep (GNU grep) 2.5.1