[plug] Help..how do I match /usr/local but not ./usr/local ?

[Lawrie Abbott]

Hi Pluggers,
I  can do

sed -i -e 's|/usr/local|/usr|g' filename

but there are instances in one of the files that  contains instances of 
"./usr/local" that I want left untouched.
I have been looking through sed info and cant find a way to do it (that 
my feeble brain can cope with). Can anyone enlighten me?  Perl?

Thanks
Lawrie