I'm pretty sure it has been discussed, but I wasn't able to find the thread.
I don't like the idea for a few reasons. First, I'm perfectly happy with a growing list of "Re"s most of the time. I'm used to it from mailing lists and such. Second, people do it differently. Some change "Re: Re:" to "Re2" some change it to "Re^2"... And third, given vanity
prefixes like "(Ovid)" or "•", it would be much harder to do correctly than your simple regular expression implies.
"My two cents aren't worth a dime.";