raises: add missing | metacharacter to is_fully_escaped and unescape

[bysiber]

is_fully_escaped checks whether a match pattern body (inside ^...$) contains only escaped regex metacharacters, so pytest knows it can treat it as a literal string for diff output. The current metacharacter list is:

metacharacters = "{}()+.*?^$[]"

This is missing |, the regex alternation operator. A pattern like ^foo|bar$ would pass the is_fully_escaped check even though the | makes it a regex alternation rather than a literal. This causes rawmatch to be set to the unescaped value foo|bar, leading to a misleading diff in the failure message.

The unescape function has the same omission in its character class, so \| wouldn't be unescaped even if someone properly escaped it.

This adds | to both the metacharacter list in is_fully_escaped and the character class in unescape.

[The-Compiler]

Similarly to your other PRs I commented on, could you please add a regression test to catch this issue?

[bysiber]

Added a regression test that verifies | is recognized as a regex metacharacter in is_fully_escaped, and that unescape properly handles escaped pipes.

[bluetech]

I tweaked the test a bit and added a changelog.

[patchback]

Backport to 9.0.x: 💔 cherry-picking failed — conflicts found

❌ Failed to cleanly apply 960ddc6 on top of patchback/backports/9.0.x/960ddc6f67d7a37b8390f260f8ded2049f8a28de/pr-14222

Backporting merged PR #14222 into main

1. Ensure you have a local repo clone of your fork. Unless you cloned it
   from the upstream, this would be your origin remote.
2. Make sure you have an upstream repo added as a remote too. In these
   instructions you'll refer to it by the name upstream. If you don't
   have it, here's how you can add it:

   $ git remote add upstream https://github.com/pytest-dev/pytest.git

3. Ensure you have the latest copy of upstream and prepare a branch
   that will hold the backported code:

   $ git fetch upstream
   $ git checkout -b patchback/backports/9.0.x/960ddc6f67d7a37b8390f260f8ded2049f8a28de/pr-14222 upstream/9.0.x

4. Now, cherry-pick PR raises: add missing | metacharacter to is_fully_escaped and unescape #14222 contents into that branch:

   $ git cherry-pick -x 960ddc6f67d7a37b8390f260f8ded2049f8a28de

   If it'll yell at you with something like fatal: Commit 960ddc6f67d7a37b8390f260f8ded2049f8a28de is a merge but no -m option was given., add -m 1 as follows instead:

   $ git cherry-pick -m1 -x 960ddc6f67d7a37b8390f260f8ded2049f8a28de

5. At this point, you'll probably encounter some merge conflicts. You must
   resolve them in to preserve the patch from PR raises: add missing | metacharacter to is_fully_escaped and unescape #14222 as close to the
   original as possible.
6. Push this branch to your fork on GitHub:

   $ git push origin patchback/backports/9.0.x/960ddc6f67d7a37b8390f260f8ded2049f8a28de/pr-14222

7. Create a PR, ensure that the CI is green. If it's not — update it so that
   the tests and any other checks pass. This is it!
   Now relax and wait for the maintainers to process your pull request
   when they have some cycles to do reviews. Don't worry — they'll tell you if
   any improvements are necessary when the time comes!

🤖 @patchback
I'm built with octomachinery and
my source is open — https://github.com/sanitizers/patchback-github-app.