Fix: is_fully_escaped does not handle consecutive backslashes correctly

[EternalRights]

Closes #14392

Description

The is_fully_escaped function in raises.py only checked whether the character immediately before a regex metacharacter was a backslash. It did not count how many consecutive backslashes preceded the metacharacter.

In regex, backslashes escape each other in pairs. So \\. (two backslashes then a dot) means an escaped backslash followed by an unescaped dot. But is_fully_escaped(r\\.) was incorrectly returning True because it only checked if s[i-1] was \.

This affects pytest.raises(match=...) error messages: when the match pattern contains escaped backslashes before a metacharacter, the function incorrectly considers the pattern fully escaped and skips showing a regex diff on match failure.

Fix: Count consecutive backslashes before each metacharacter. Even count means the metacharacter is not escaped; odd count means it is.

Type of Change

• Bug fix

Checklist

• Added tests for changed functionality
• Added changelog entry
• No documentation update needed (internal helper function)

[alokshukla631]

This looks good to me. Counting the run of preceding backslashes matches regex escaping rules much better than the old single-character check, and the added cases make the intended behavior easy to follow.

[EternalRights]

@alokshukla631
Thank you for your review and feedback!
I've updated the PR branch to stay in sync with the main branch, and I'm now awaiting approval from a maintainer with write access.
If you have any further suggestions, feel free to let me know.

[bluetech]

Can we use a regex for it instead? I'm thinking we can do it in 2 steps:

1. Strip all escapes (re.sub(r'\\.', '', pattern))
2. Check if still contains any metacharacters (simple containment check).

Step 1 should ensure that none of the chars found in step 2 are escaped.

I think that would be shorter and a bit more obvious.

[EternalRights]

Thanks for the review! Updated the changelog and refactored is_fully_escaped to use the regex approach — it's cleaner this way.

[bluetech]

Thanks

[patchback]

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

❌ Failed to cleanly apply a38b20d on top of patchback/backports/9.0.x/a38b20d79039d50c504320b5d4d790c61893a90e/pr-14393

Backporting merged PR #14393 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/a38b20d79039d50c504320b5d4d790c61893a90e/pr-14393 upstream/9.0.x

4. Now, cherry-pick PR Fix: is_fully_escaped does not handle consecutive backslashes correctly #14393 contents into that branch:

   $ git cherry-pick -x a38b20d79039d50c504320b5d4d790c61893a90e

   If it'll yell at you with something like fatal: Commit a38b20d79039d50c504320b5d4d790c61893a90e is a merge but no -m option was given., add -m 1 as follows instead:

   $ git cherry-pick -m1 -x a38b20d79039d50c504320b5d4d790c61893a90e

5. At this point, you'll probably encounter some merge conflicts. You must
   resolve them in to preserve the patch from PR Fix: is_fully_escaped does not handle consecutive backslashes correctly #14393 as close to the
   original as possible.
6. Push this branch to your fork on GitHub:

   $ git push origin patchback/backports/9.0.x/a38b20d79039d50c504320b5d4d790c61893a90e/pr-14393

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!

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