Let me slide into this review to restate my firm opposition from the outset to the valiant efforts of the proposal authors to support the 'bare' /regex/ syntax in Swift and my surprise to see it being brought to review in the proposal. It simply is not worth the known lexical contortions and source breaking of popular open source projects it would inflict on the language. At best it could be presented as a "future direction" for when these issues have been thought through. I just don't see #/regex/# being much less recognisable or "clean" as an alternative "term of art" or the need to support both.
Not strictly relevant to the review, given that ( and ) are special characters important to regex syntax I don't see a syntax like #re(stuff) being much more promising. Any literal for something as subtle as a regex terminated by a single character simply isn't going to cut it.