What is the regex for searching a word only if it is in All Capital letter?

I have a string who did DID what ? what can I add to the below regex expression to search only if DID is in capital letters ?

output =  re.findall(
            r"did", # not adding DID here as the output maycome with did or DID but i need to check only when its all caps
            searchstring,
        )

>Solution :

You can find all CAPS letter word this way:

import re

txt = "who did DID what Did DiD?"
result = re.findall(r"\b[A-Z]+\b", txt)
print(result)

Output:

['DID']