grep: why doesn't ':alpha:' work

[Matthew Woehlke]

While trying to reply to swiftguy's question, I noticed something odd...

$ echo foo | grep '[:lower:]'
foo
$ echo foo | grep '[:alpha:]'
$ echo FOO | grep '[:alpha:]'
$ echo FOO | grep '[:upper:]'
$ locale
LANG=en_US.UTF-8
LC_CTYPE="en_US.UTF-8"
LC_NUMERIC="en_US.UTF-8"
LC_TIME="en_US.UTF-8"
LC_COLLATE="en_US.UTF-8"
LC_MONETARY="en_US.UTF-8"
LC_MESSAGES="en_US.UTF-8"
LC_PAPER="en_US.UTF-8"
LC_NAME="en_US.UTF-8"
LC_ADDRESS="en_US.UTF-8"
LC_TELEPHONE="en_US.UTF-8"
LC_MEASUREMENT="en_US.UTF-8"
LC_IDENTIFICATION="en_US.UTF-8"
LC_ALL=
$ echo foo | LC_ALL=C grep '[:alpha:]'
$ echo FOO | LC_ALL=C grep '[:alpha:]'
$ echo FOO | LC_ALL=C grep '[:upper:]'
$ grep --version
grep (GNU grep) 2.5.1
[snip]

Am I doing something wrong...?

--
Matthew
Caution: keep out of reach of adults.