@rxwei I have a lot of tests cases in this PR if you'd like to take a look. I'm not entirely sure what behavior we want for all of them.

Literal capture structure tests look good to me.

Did you plan to add DSL capture tests as well? For example, the following DSL

let regex = Regex {
    oneOrMore {
        capture("a")
        capture("b")
    }
} // `Regex<(Substring, [(Substring, Substring)])>`

is expected to produce Regex<(Substring, [(Substring, Substring)])> by the current design.

Unrelated to this PR: By the current DSL design, we'll have a .flatten() method which will be generated by the converter when converting literal (?:(a)(b))+.

let regex = Regex {
    oneOrMore {
        capture("a")
        capture("b")
    }.flatten()
} // `Regex<(Substring, [Substring], [Substring])>`

is expected to produce Regex<(Substring, [(Substring, Substring)])>

What is matchResult.1?

[(Substring, Substring)]

@swift-ci please test linux platform