[iso8859-1]

This is what I get:

    $ echo HI | LANG=C grep '^[a-z]'
    $ echo HI | LANG=en_US.utf8 grep '^[a-z]'
    $ 

How come?

[p9idf]

I was able to reproduce the bug. It could be a version thing.

  ; grep --version
  GNU grep 2.6.3
  ; echo A | LANG=en_US.utf8 grep '[a-z]'
  A

[Ives]

No, I have the same version but not a similar result. I also have the en_US.utf8 locale installed.