OK. Lot of thanks for your explanations and your work to fix that. I tried a solution without regex and it works!

@filosfino Can you try again? It should be fixed now.

@g964
I didn't realize this problem when a user can optionally use some crate and I don't think there's a way to completely fix it.
The workaround was to bind to a different identifier.

#[cfg(test)] extern crate regex as t_regex;
#[cfg(test)]
mod tests {
    use super::*;
    use t_regex::Regex;
}

You can also move extern crate inside mod tests and do use self::regex::Regex;. I don't know which one is better. If Rust users have opinions, please let me know.

Another workaround might be to avoid use regex::Regex; and just do regex::Regex::new in the reference solution.

OK, I'll try and let you know.

I am lost:-( Maybe it would be better if you fix it!

@g964 I think it validated because your solution contains the declaration.
To use crates in test, do

#[cfg(test)]
extern crate regex; // <- outside of `mod tests`

#[cfg(test)]
mod tests {
}

I'll try fixing it if you want.

@filosfino

See Rust wiki page for the list of crates.

For self::, it's because Rust 1.15 is a huge hack on Codewars for historical reasons. It's fixed on 1.25/1.33. See List of Rust 1.15 kata to update. (this was caused by invalid test code)

I will try again...
Sorry for the inconvenience!

Fixed, thanks!