Yes, RE is a valid Scrabble word!
in the matter of (used typically as the first word in the heading of an official document or to introduce a reference in a formal letter)
Synonyms: about, concerning, regarding, with regard to, relating to, apropos (of), on the subject of, respecting, in respect of, with respect to
the chemical element rhenium
(in tonic sol-fa) the second note of a major scale
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