I edited the language and the examples. In particular, I used the Bob example from SIP-11. Since "big_bob" makes an appearance in SLS 1.1, I'm pretty sure Bob is significant in some way.
The gist is that arbitrary identifiers are accepted for "simple form" or braceless interpolation, including backquoted identifiers.
The fact that identifiers with dollars must be backquoted is mentioned in the commit message but not yet in the SIP document.
The implementation is athttps://github.com/som-snytt/scala/commit/85316634970e87b925e18160cefbfeef74d728b6
I just noticed it incorrectly excludes s"$_foo". Off the top of my head, I don't know whether it's better to require s"$`_`foo" or s"$`_foo`"; probably the first.
Also, scanning for ids uses nextChar instead of nextRawChar, which differs in newline handling, but I haven't experimented yet. I'm just on lunch break (at the home office, watching company videos, for the record. I just needed a relaxing coding break.)