grep regex whitespace behavior
This looks like a behavior difference in the handling of \s between grep 2.5 and newer versions (a bug in old grep?). I confirm your result with grep 2.5.4, but all four of your greps do work when using grep 2.6.3 (Ubuntu 10.10).
Note:
GNU grep 2.5.4
echo "foo bar" | grep "\s"
(doesn't match)
whereas
GNU grep 2.6.3
echo "foo bar" | grep "\s"
foo bar
Probably less trouble (as \s is not documented):
Both GNU greps
echo "foo bar" | grep "[[:space:]]"
foo bar
My advice is to avoid using \s ... use [ \t]* or [[:space:]] or something like it instead.