I’m trying to find a way to use syntax/parse to match square brackets, like [ and ]. I’d like to match, for example, syntax like (a [foo]) but not (a (foo)). Maybe I’m asking too much?

That can be done. See the syntax property 'paren-shape

Alexis has a nice library that queries the syntax property for you already: https://docs.racket-lang.org/syntax-classes/index.html#%28form._%28%28lib._syntax%2Fparse%2Fclass%2Fparen-shape..rkt%29._~7ebrackets%29%29

awesome, thank you!