You may use this regex with a lookahead:
[A-Z]+(?:s+[A-Z]+)*(?=[^A-Z]*$)
RegEx Demo
RegEx Details:
: Word boundary
[A-Z]+
: Match a word with 1+ uppercase letters
(?:s+[A-Z]+)*
: Match 0 or more space separated uppercase words
(?=[^A-Z]*$)
: Lookahead to assert that we don't have any uppercase letters ahead
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