I just did 'strings /dev/urandom > stuff' for 100M. Got it in memory first with untimed grep, then: $ time grep -i blah stuff bLAH blAH real 0

I just did 'strings /dev/urandom > stuff' for 100M. Got it in memory first with untimed grep, then: $ time grep -i blah stuff bLAH blAH real 0m7.227s user 0m7.194s sys 0m0.011s $ time LANG=C grep -i blah stuff bLAH blAH real 0m0.486s user 0m0.467s sys 0m0.016s So a pretty big difference. My default lang is en_US.utf8.