Table Design - Vaildation Rule

[MUKESHY12390]

Hi All,
I'm trying to create validation rule where only value in upper cases "MUKESH" would be allowed in field.
this is what I'm trying with - Is Null OR Not Like "*[!a-z]*"
but not seem to be work .

Note : I know I can format field with ">", so that whatever user enters it will turn into UPPER case., but this is not something I'm looking for.

Any help would be appreciate.

Regards,
Mukesh Y