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
each of the lines in which light (and heat) may seem to stream from the sun or any luminous body, or pass through a small opening
Synonyms: beam, shaft, streak, bar, pencil, finger, stream, gleam, flash, glint