[pattern matching] "invalid" left-bracket should match itself, but does not

[avih]

In shell pattern matching, a [ which does not start a valid bracket expression should match itself.

This was first introduced in POSIX 2008:

If an open bracket introduces a bracket expression ... . Otherwise, '[' shall match the character itself.

And made more explicit in POSIX 2024:

A (left-square-bracket) shall introduce a bracket expression if the characters following it meet the requirements for bracket expressions... . A (left-square-bracket) that does not introduce a valid bracket expression shall match the character itself.

(it's not mentioned in POSIX 2004)

This Applies to globs, as well as case patterns and parameter expansion. Tested in release 93u+m/1.0.10+f0999ab7:

$ x=[foo
$ touch "$x"; echo [* \[*
[* [foo
$ case $x in [*) echo OK;; \[*) echo BAD; esac
BAD
$ echo "${x#[?}" "${x#\[?}"
[foo oo
$

Most other shells do adhere, including bash, dash, mksh, busybox-ash, yash, net/free bsd shell, oil shell.

Not adhering: all variants of AT&T ksh which I have, pdksh and openbsd shell, dash up to 0.58, Schily (RIP) bosh.

FWIW, I imagine most people would quote it rather relying on this behavior, but still.