Support \u Unicode escape

[roblourens]

Ripgrep supports \x{1234}, but not \u1234, while many people expect the latter. Have you considered also supporting \u?

If not, we'll rewrite one to the other for compatibility with the JS regex engine.

Ref: microsoft/vscode#39404

[BurntSushi]

This is an interesting ticket. It probably belongs as an issue on the regex engine itself. Historically, I chose not to support the \u syntax because Rust itself has support for such syntax in its string literals. But obviously that isn't available when you aren't writing Rust code. That seems like a good enough argument to support the syntax to me.

Note that technically, according to UAX#18, the \x syntax satisfies the requirement, but they also list a specific grammar for the \u (and \u{...}) style of syntax.

I filed a ticket with the regex crate, but I'll leave this open so it's easy to track with respect to ripgrep.

(I don't have a specific timeline on this. I should be able to get it done as part of the regex-syntax rewrite that I'm working on, but I don't have an ETA. It could be a few weeks (ideal) if things go well, or it could be months.)