[feature or defect]: no `\(` when opening a new group in group path

[tachib-akiko]

It sounds a little confusing, so I'm going to interpret it with a valid and invalid path:

type	path
Valid:	/phim/:season/:chapName(?:(.+\)-)?:chap(\d+)?
Notvalid:	/phim/:season/:chapName(?:\(.+\)-)?:chap(\d+)?

as soon as I add \ first ( to mark this as a group, vue-router error message cannot identify the group

Demo: https://paths.esm.dev/?p=AAMeJRyAwQIWLALcM-BMgWII2IPZDhsDpEHIARpvUADEDwGVDdAOQJSA0QJRt1IlE8DcJkEBAAA.&t=/phim/sfweferg/tap-432#

I don't know if this is a feature or a bug, but if it's a feature, please add it as noted in the document. This is kind of confusing for people who use regexp like me.

[posva]

This is expected: it's part of the escaping characters, it's a confusing topic on itself but to make it short: \) allows to escape the closing parenthesis while \( is going to pass that \ to the regexp because the ( doesn't need to be escaped.
This is an advanced feature that deals with some internals so it's intentionally not documented to not confuse users further.