Sort: problem with option -f

[Guido Milanese]

I am facing a problem concerning SORT on a Linux Ubuntu 9.04
distribution.
I noticed that the -f option seems not to work. This is an example:
DOG
CAT
zebra
dog
ZEBRA
mouse

Running "sort" on the file I get:
CAT
dog
DOG
mouse
zebra
ZEBRA

Running "sort -f " I get the same result. No problem of encoding, being
plane letters <127 -- to be sure I saved the list as 'test', and "file 
test" says it's a pure ASCII text. Without the '-f' option, 'sort' used to
distinguish among lowercase / uppercase.
Is this a bug or a new behaviour of 'sort' ?

Thanks!
guido
guido.milanese@unicatt.it 
www.arsantiqua.org