-I ignores too many lines

[Shaun Jackman]

-I seems to be ignoring lines that don't match the regexp. I'm sorry 
if this turns out to be something I've done wrong (as will probably 
be the case), but it seems pretty cut and dry.

The desired output of the following script is
+dog
while ignoring the line with only white space. (not the blank line in 
b has white space in it, which is why I can't simply use -B). In my 
test, diff produces no output.

Thanks,
Shaun

diff - GNU diffutils version 2.7

cat > a <<EOF
hello
bob
EOF
cat > b <<EOF
hello
        
bob
dog
EOF
diff -uI "^ *$" a b