Apparently the word “re” by itself can be used to mean “regarding”, and it actually is not a shortening of the word “regarding”, but rather the Latin word “re” meaning “thing, matter, subject” used in legalese. I was just using the shorthand “re.” for “regarding” that I’ve sometimes seen written, but apparently “re” alone is fine. The writer David Foster Wallace often used “w/r/t” in his writing (meaning “with regard to”). After reading a few of his essays, I starting using that sometimes too. For some reason the sense of “regarding” is something we seek abbreviations for.