Semver compatibility failure

[hashedone]

What version of regex are you using?

1.8.1, tried 1.8.0 as well

Describe the bug at a high level.

The 1.8.0 is semver incompatible with 1.7.0.

The tracing-subscriber crate is depending on regex (when env-filter is enabled) to parse the env config string. It is depending on the ^1 version, so it picks 1.8.1 right now. Any trail of creating EnvFilter ends in a panic due to an error in some regex parsing.

What are the steps to reproduce the behavior?

The minimal example (this output is literally from this one): https://github.com/hashedone/regex-incompat-failure

This behaves the same when I fix the regex version to 1.8.0 with cargo update -p regex --precise 1.8.0, but is "fixed" when I go to 1.7.3: `cargo update -p regex --precise 1.7.3"

What is the actual behavior?

thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: Syntax(
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
regex parse error:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 1: (?x)
 2:             ^(?P<global_level>(?i:trace|debug|info|warn|error|off|[0-5]))$ |
                                      ^
 3:                 #                 ^^^.
 4:                 #                     `note: we match log level names case-insensitively
 5:             ^
 6:             (?: # target name or span name
 7:                 (?P<target>[\w:-]+)|(?P<span>\[[^\]]*\])
 8:             ){1,2}
 9:             (?: # level or nothing
10:                 =(?P<level>(?i:trace|debug|info|warn|error|off|[0-5]))?
11:                     #          ^^^.
12:                     #              `note: we match log level names case-insensitively
13:             )?
14:             $
15:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
error: Unicode-aware case insensitivity matching is not available (make sure the unicode-case feature is enabled)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
)', /home/hashed/.cargo/registry/src/gb.xjqchip.workers.dev-1ecc6299db9ec823/tracing-subscriber-0.3.16/src/filter/env/directive.rs:140:10
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

What is the expected behavior?

No panic should occur. Some functionality got changed, causing regex to be previously accepted and now be refused, which seems like a breaking change.

[BurntSushi]

Duplicate of #982

This isn't a semver issue but a result of tracing-subscriber under specifying their dependency on regex. It "happened" to work in regex 1.7 because of Cargo feature unification.

See my comment here: #982 (comment)