> You are depending on American Nerd concepts and you should set LANG=C.

   Perhaps.  Or, perhaps not.

> Yes but this will cause the opposite problem.  You'll not be matching,
> when you think you should.  (Unless you are using -v for non-matching
> logic.)

   As usual, many things are possible, including spurious matching, even
without an exotic locale.  For example:

      dyi# echo ${LOGNAME} | grep -q [a-z]
      dyi# echo $?
      0
      dyi# echo ${LOGNAME} | grep -q [A-Z]
      dyi# echo $?
      0

   Revealing more:

      dyi# set -x
      dyi# echo ${LOGNAME} | grep [a-z]
      + echo root
      + grep a b
      a
      dyi# echo ${LOGNAME} | grep [A-Z]
      + echo root
      + grep A B
      A

      dyi# set +x
      dyi# ls -l ?
      -rw-r--r--   1 root       sys              0 Apr 14 09:05 A
      -rw-r--r--   1 root       sys              2 Apr 14 09:06 B
      -rw-r--r--   1 root       sys              0 Apr 14 09:03 a
      -rw-r--r--   1 root       sys              2 Apr 14 09:04 b

      dyi# cat B
      A
      dyi# cat b
      a

   Given enough arguments, grep doesn't have much interest in stdin, so
these results don't depend on ${LOGNAME}:

      dyi# set -x
      dyi# grep [a-z]
      + grep a b
      a
      dyi# echo $?
      + echo 0
      0
      dyi# grep [A-Z]
      + grep A B
      A
      dyi# echo $?
      + echo 0
      0

   Everything's complicated, but, as shown before, quoting the RE solves
problems of this type.