"Unrecognized escape sequence" in Clippy

[ThomasdenH]

I opened rust-lang/rust-clippy#4170 and was to open an issue here. The escaped backslash is not recognized:

error: regex syntax error: unrecognized escape sequence
   --> src/compiler/utilities.rs:187:52
    |
187 |         static ref RE: Regex = Regex::new("^..?($|[\\/])").unwrap();
    |                                                    ^^
    |
    = note: #[deny(clippy::invalid_regex)] on by default
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#invalid_regex

[ThomasdenH]

Apparently it needs to be escaped twice, which makes sense. I kept the issue open there for a better warning.

[BurntSushi]

Glad you figured it out. Note that the first example in the docs specifically mentions this issue. In particular, you might consider using raw strings.