Too many people love string interpolation to remove it, and, with the form I have it in StringLiterals.jl to unify literals and interpolation, without causing problems with compatibility with other C-like language string literals (i.e. ONLY \ and " are special and need to be quoted, and what follows \ is pretty much the same between languages, but people understand that there may be some differences in the escape sequences).
What about my idea, which costs only a single character if you have a lot of string interpolation of simple identifiers (frankly, I find that’s rare for my uses now, because usually I want to have a bit of formatting), is used to mark interpolated strings in other languages (and gives an indication to the user that this is NOT your normal C-like string literal), i.e. $"This is my interpolated string, with $foo, $bar, $baz" ?
$"..." formatted strings could accept both legacy style interpolation (possibly with Stefan’s whitelist/blacklist idea, although I might simply say that you make it easy to explain, and say that the identifiers must be ASCII (or at least Latin1) (and so not need loaded tables, and avoid the dependence on a specific version of Unicode that I had brought up - the code would be easier to read internationally, only if you had identifiers with characters > 0xff would you need to use either $(...) or \(...)).
I know that @stevengj said he didn’t buy the “LaTeX” arguments, but from his own documentation of https://github.com/stevengj/LaTeXStrings.jl, I think that this would actually be very useful to people using his package.
You can also use the lower-level constructor latexstring(args...), which works much like string(args...) except that it produces a LaTeXString result and automatically puts $ at the beginning and end of the string if an unescaped $ is not already present. Note that with latexstring(...) you do have to escape $ and \: for example, latexstring("an equation: \$1 + \\alpha^2\$"). One reason you might want to use latexstring instead of L"..." is that only the former supports string interpolation (inserting the values of other variables into your string).
If this change were made, then only the \ would need to be escaped, which any LaTeX users from any of the languages with C-like string literals would be familiar with (and NOT familiar with the need to escape $!)
So, his example would become simpler: latexstring("an equation: $1 + \\alpha^2$"), and there are no problems using string interpolation using the Swift ‘(…)’ syntax.
I think that could be added very quickly to the FemtoLisp parser, and I already have the code for the rest (the only difference is that instead of f"..." and F"...", it would be "..." and $"...")