[Bug 2007155] [NEW] grep: sometimes mistakenly matches lines when last of multiple patterns includes backref

[Santiago Ruano Rincon]

Public bug reported:

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)

** Affects: grep (Ubuntu)
     Importance: Undecided
         Status: New

** Patch added: "1029235-grep-bug-backref-in-last-of-multiple-patterns.patch"
   https://bugs.launchpad.net/bugs/2007155/+attachment/5646814/+files/1029235-grep-bug-backref-in-last-of-multiple-patterns.patch

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https://bugs.launchpad.net/bugs/2007155

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

Status in grep package in Ubuntu:
  New

Bug description:
  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)

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