使用漢語環境時，grep命令竟然不區分大小寫

[root@redhat ~]# export LANG=zh_CN.GB18030
[root@redhat ~]# grep [A-Z]om law
Anyway, Tom, I need your help. I'd like to make the test up
think you could help me? After work, about 7 PM,come to

[root@redhat ~]# export LANG=zh_CN.UTF-8
[root@redhat ~]# grep [A-Z]om law
Anyway, Tom, I need your help. I'd like to make the test up
think you could help me? After work, about 7 PM,come to

[root@redhat ~]# export LANG=en_US

[root@redhat ~]# grep [A-Z]om law
Anyway, Tom, I need your help. I'd like to make the test up