Looks good to me, except I suspect the string pattern is wrong. How do you output a string that contains a double quote? Could you add that to an examplefile, and adjust the regex to match? Or if you'd prefer, point me to docs and I'll adjust after merging.
Unusually, the string literals in Stan do not allow for double quotes within the string or even "\". So the the regex I use is even more permissive than the parser, but I kept it simple to avoid specifying the full set of characters that are allowed within the string. The string in the example file is in fact the only characters that are allowed in a valid string.