I'm confused by this example in the README.md, I don't think the 2nd parameter matches the proposed spec text:

RegExp.prototype[Symbol.customMatcher] = function(subject, {matchType}) {
    const result = this.exec(subject);
    if(matchType == "boolean") return result;
    if(matchType == "extractor") return [result, ...result.slice(1)];
}

Isn't the 2nd argument just a string, and aren't the possible values "boolean" and "list"?

Also, I'm not sure I agree with the result for a RegExp custom matcher as suggested here: https://github.com/tc39/proposal-pattern-matching?tab=readme-ov-file#regex-extractor-patterns

To match against a RegExp, you may want to match against either named capture groups, positional capture groups, or the whole match. I think the matcher could just be:

RegExp.prototype[Symbol.customMatcher] = function(subject, hint) {
    const result = this.exec(subject);
    if (hint === "boolean") return !!result;
    if (hint === "list") return result && [result];
}

And the pattern would look like:

when /(?<left>\d+) \+ (?<right>\d+)/({groups:{let left, let right}}): ...;
when /(\d+) \* (\d+)/([void, let left, let right]): ...;
when /\d+/([let digits]): ...;

I'm in the middle of writing some code using C#'s pattern matching and ran into an interesting mechanism. C# has an object initialization syntax that lets you write code like this:

var a = new Foo() {
  Bar = 1,
  Baz.Quxx = 2,
}

which amounts to

var a = new Foo();
a.Bar = 1;
a.Baz.Quxx = 2;

and { Baz.Quxx = 2 } is essentially the same as { Baz = { Quxx = 2 } }.