How does this handle backreferences? The resolved field points to the referenced capturing group, but this is no longer unique.

Thank you for letting me know! I think we need to change the node.
That should be treated as a breaking change, right?

That should be treated as a breaking change, right?

Maybe. You currently implemented resolved: CapturingGroup[] which is a breaking change. However, we could also use the same trick you used for Unicode set character classes in v-flag regexes. So we could do something like this:

type Backreference = AmbiguousBackreference | UnambiguousBackreference
interface BaseBackreference {
  type: "Backreference"
  // other props
  ambiguous: boolean
  resolved: CaptruingGroup | CaptruingGroup[]
}
interface AmbiguousBackreference extends BaseBackreference {
  ambiguous: true
  resolved: CaptruingGroup[]
}
interface UnambiguousBackreference extends BaseBackreference {
  ambiguous: false
  resolved: CaptruingGroup
}

The interface/property names could use some work, but I hope I communicated the idea. This wouldn't be a breaking change, because regexes without duplicate group names produce the same AST as before (except for the additional ambiguous: false property).

However, I'm not sure whether this AST is ideal. It's backwards compatible, but it's also a bit more tricky to understand.

Thank you! That's a good idea!
I think it's a good thing that it avoids breaking changes since it is additional information that is not related to the AST.