Match all characters enclosed in brackets

I have trouble finding the correct regex.

I know that a sequence of characters enclosed in brackets means their optional:
[xyz] means any 'x' OR 'y' OR 'z'

but how can I find them all?
any 'x' AND 'y' AND 'z' in whatever sequence and quantity

p.e.
Match first an uppercase character then all lowercase characters with at least 1 uppercase character
This would be easier if the sequence of characters enclosed in brackets aren't optional.

Something like this:
\u[a-zA-Z]




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