Panic "Indexing into empty node"

[jDomantas]

This code:

use logos::Logos; // logos 0.12.1

#[derive(Logos)]
enum Token {
    #[regex("x()*")]
    What,
    #[error]
    Error,
}

Yields this compile error:

error: proc-macro derive panicked
 --> src\lib.rs:3:10
  |
3 | #[derive(Logos)]
  |          ^^^^^
  |
  = help: message: Indexing into an empty node. This is a bug, please report it at:

          https://github.com/maciejhirsz/logos/issues

error: could not compile `logos-bug` due to previous error

rustc version rustc 1.66.0 (69f9c33d7 2022-12-12)

[jeertmans]

Maybe that’s a bug, but your are trying to capture something that is empty. If you need to capture parentheses, you first need to escape them.

[jDomantas]

I know that this particular regex is not something I want to write, I'm just reporting this because it's definitely a bug in the macro. For valid regexes it should compile, and for invalid ones it should show a reasonable error message (or at least point to the problematic regex), and in this case it is doing neither. Even better, the macro itself asks me to report this.