The bids specification allows for upper case labels for subjects:
BIDS uses custom user-defined labels in several situations (naming of participants, sessions, acquisition schemes, etc.) Labels are strings and MUST only consist of letters (lower or upper case) and/or numbers.
However when i use upper case e.g sub-X01, the bids validator (http://bids-standard.github.io/bids-validator/) says error 1 “NOT_INCLUDED” and if i use sub-x01 then it passes. I think in this regard, the bids validator is therefore inconsistent with bids specification…