gawk ignores case with LANG=en_US

[Jim Keniston]

--- bug.awk ---
/^[a-z]/ { print }
--- input ---
1
a
A
--- output_buggy ---
a
A
--- output_expected ---
a
-----
Repeat by:
$ gawk -f bug.awk < input

Assuming that environment variables LC_ALL and LC_CTYPE are
undefined, if I run the above with the LANG environment variable
set to "en_US.utf8" or "en_US", "A" matches "^[a-z]" and the
output is as in output_buggy.  Setting IGNORECASE=0 in the
command line or the script doesn't help.

If I do
$ LANG= gawk -f bug.awk < input
I get the expected output.

gawk version: GNU Awk 3.1.5
OS: RH Fedora 9 + Linux v2.6.29-rc8

Jim Keniston
IBM Linux Technology Center
Beaverton, OR