grep: sometimes mistakenly matches lines when last of multiple patterns includes backref

[Santiago Ruano Rincon]

This bug was introduced upstream with grep 3.4. Originally reported in Debian: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1029235#19

On an ubuntu focal machine:
  echo 'Total failed: 2 (1 ignored)' | grep -e '^Total failed: 0$' -e '^Total failed: \([0-9]*\) (\1 ignored)$'
  Total failed: 2 (1 ignored)

A fixed grep doesn't match the line.

This has been fixed in debian unstable with 3.8-4, and in bullseye with 3.6-1+deb11u1.

I've tested the attached patch in ubuntu focal (grep 3.4)

[Ubuntu Foundations Team Bug Bot (crichton)]

The attachment "1029235-grep-bug-backref-in-last-of-multiple-patterns.patch" seems to be a patch. If it isn't, please remove the "patch" flag from the attachment, remove the "patch" tag, and if you are a member of the ~ubuntu-reviewers, unsubscribe the team.

[This is an automated message performed by a Launchpad user owned by ~brian-murray, for any issues please contact him.]