Piped grep giving different results in "M-x grep" vs. from a shell ??

[François Granade]

I'm trying to grep for the string "bar" but not for "foobar".

From a shell, if I run "grep -nH -e 'bar' * | grep -v 'foobar'", I get the 
results I expect: the lines that contain "bar" but not 'foobar'.

However, if I run from within emacs (M-x grep), the same command returns also 
the lines that contain "foobar". 

I'm running emacs shipped in MacOS X 10.6 "GNU Emacs 22.1.1 (mac-apple-darwin) 
of 2010-02-26 on crevasse.apple.com"; if I'm correct, pipes should be supported 
in this version of Emacs. What's wrong with me ?

Thanks a lot for any help - I feel I'm becoming crazy :)

Faria