Thanks for exploring. I expected it might end up in a mess like this, but I appreciate you at least thinking it over!
Lest it be lost in the din, my original concern was not about paralleling string syntax per se, but about this footgun:
Somebody mentioned upthread the idea of making “ignore whitespace / extended mode” a separate flag, orthogonal to the regex spanning multiple lines. Not sure that makes sense either. (Should a non-extended-mode regex match embedded newlines? What about indentation, then?). Still, perhaps worth a moment of consideration in the post-review followup.
In practice, I hope that the community will generally favor the builder DSL over multiline regex literals, making this is a relatively rare footgun.
I'll ultimately trust the core team's judgement on this question. Thanks again for navigating this epic barrage of questions.