[R] negation in grep

[orzack]

Does anybody know how to negate a string in a grep command, i.e., 
what I need is to return only strings that do NOT contain a second 
string anywhere in the entire string.


for

txt <- c("boo","goo","doob","foo","boofoo")

I need a grep command that returns strings with "oo" except when "b" 
is present anywhere. I know that

grep("[^b]oo" would work to exclude "boo" and "boofoo" but not "doob"

So, what I need is

grep("oo + some syntax", txt)

should return

goo doo foo

I have tried

  grep("oo!boo",txt)

but it returns character(0).

any suggestions are much appreciated!

S.
-- 
Steven Orzack