Grep's backref-alt test still gets an assertion failure when using glibc-2.27-32.fc28 (and many others), but I have reduced it to something much more readable in the attached. I've also updated to latest gnulib.
This demonstrates that the reduced regexp triggers the same error, in both grep and sed: $ printf z > in && LC_ALL=C sed -r 's/(()x)|\2//' in sed: regexec.c:1342: pop_fail_stack: Assertion `num >= 0' failed. zsh: abort (core dumped) LC_ALL=C sed -r 's/(()x)|\2//' in [Exit 134 (ABRT)] $ printf z > in && LC_ALL=C grep -E '(()x)|\2' in grep: regexec.c:1342: pop_fail_stack: Assertion `num >= 0' failed. zsh: abort (core dumped) LC_ALL=C grep --color=auto -E '(()x)|\2' in [Exit 134 (ABRT)]
0001-grep-test-and-gnulib.diff
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