Arch Linux

Intended change with grep 3.8 from [1]

Regular expressions with stray backslashes now cause warnings, as their unspecified behavior can lead to unexpected results. For example, '\a' and 'a' are not always equivalent <https://bugs.gnu.org/39678>. Similarly, regular expressions or subexpressions that start with a repetition operator now also cause warnings due to their unspecified behavior; for example, *a(+b|{1}c) now has three reasons to warn. The warnings are intended as a transition aid; they are likely to be errors in future releases.
Regular expressions like [:space:] are now errors even if POSIXLY_CORRECT is set, since POSIX now allows the GNU behavior.

[1] https://savannah.gnu.org/forum/forum.php?forum_id=10227

Likely a grml-zsh-config issue.

Very wild untested guess line 897 of /etc/zsh/zshrc from grml-zsh-config has a grep with \0 in the expression
[[ -r /etc/hosts ]] && [[ "$NOETCHOSTS" -eq 0 ]] && : ${(A)_etc_hosts:=${(s: :)${(ps:\t:)${${(f)~~"$(grep -v '^0\.0\.0.\0\|^127\.0\.0\.1\|^::1 ' /etc/hosts)"}%%\#*}##[:blank:]#[^[:blank:]]#}}} || _etc_hosts=()

Not impacted with my own zsh configuration.
I can delay for a few days the move from [testing] until we figure have a patch into grml.

https://github.com/grml/grml-etc-core/commit/e27d36095d863e19f2d50cbf6423607518778df5

Thanks, loqs and Scimmia. That typo correction did the trick in the meanwhile.