I'll be glad if Character(grapheme-cluster) semantics can be applied.
On the other hand, I agree with @xwu's indication because I also feel such feature would confuse some folks especially from other languages.
For example, some libraries such as PCRE, Oniguruma, etc. that are widely used have an expression to specify Script name such as \p{Hiragana}.
A scalar has a property of Script, but Character doesn't.
Some people may want to write /\p{Hiragana}/, however, it cannot be applied to String, but only to String.UnicodeScalarView.
They might think "I just want to write an usual Regular Expression and want it to be applied to String."
I'm looking forward to the result. But, I think it is important that which semantics is the majority rather than the fact that there are differences.
We may have to discuss first whether such feature in Swift should be called Regex or another appropriate name, and whether or not it should have similar appearance with existing regular expressions in other languages. 