Someone recently made this comment on LinkedIn: I’m so excited re: the below. (The colon may or may not have been there.)
Why not I’m so excited about this instead of the commenter’s mish-mash of somewhat immature enthusiasm and the lawyer jargon of re and the below?
If you think carefully when you write, something simple like it or this sounds much more natural than the above, the below, the latter, the former – which all sound stiff and pompous.
Dr Johnson counselled against these constructions, saying ‘As long as you have the use of your tongue and pen, never, sir, be reduced to that shift’ – shift being an eighteenth-century expression for a shabby expediency or forced measure.
And re — do we really need to use this vestigial piece of Latin? While it is usefully concise, if you’re composing something like a tweet, re has a definite air of ‘I’m using a word I would never have uttered before I went to law school and I’m only saying it now in order to sound like the real lawyer you may not think I am’.
So, to be dispensed with.
Re is a funny word, too, as it doesn’t quite mean ‘about’ or ‘regarding’ (or didn’t originally).
It is one of the grammatical forms (the ablative case, to be precise) of the Latin noun res, which means ‘thing’ — and, by extension, ‘subject’, ‘matter’, ‘affair’.
In law reports, In re sometimes appears in a style of cause, especially where the litigation is an investigation into the property of a bankrupt or the will of a testator. A famous example from Ontario is In re Estate of Charles Miller, Deceased,  OR 382 (CA). The name of that case might also be given as just Re Miller.
Use of re as a stand-in for ‘about’ goes back a long way (1707 is the earliest example in the Oxford English Dictionary), but try to do without it.
Try, particularly if your legal writing is already peppered with random bits of Latin like per and the dreadful commercial English of the nineteenth century (I acknowledge receipt of your letter, same being forwarded to our client for review). You are only alienating non-lawyer readers and sounding like some scrivener in a bad Victorian novel.
In the heading of your memo, Re: can easily be replaced by Subject:, which your reader may find more illuminating anyway.