'case' pattern matching bug with bracket expressions

[Martijn Dekker <martijn@inlv.org>]

While writing a cross-platform shell library I've come across a bug in
the way zsh (in POSIX mode) matches patterns in 'case' statements that
are at variance with other POSIX shells.

Normally, zsh considers an empty bracket expression [] a bad pattern
while other shells ([d]ash, bash, ksh) consider it a negative:

case abc in ( [] ) echo yes ;; ( * ) echo no ;; esac

Expected output: no
Got output: zsh: bad pattern: []

This is inconvenient if you want to pass such a bracket expression in a
parameter or variable, e.g. ["$param"]. If the parameter or variable is
empty, a 'bad pattern: []' error is produced.

I'm not sure whether the above is a bug or a variance in behaviour
permitted by POSIX, though of course as a writer of cross-platform shell
programs I'd prefer it if zsh acted like the majority.

However, I'm quite sure the following is a serious bug.

The same thing does NOT produce an error, but a false positive (!), if
an extra non-matching pattern with | is added:

case abc in ( [] | *[!a-z]*) echo yes ;; ( * ) echo no ;; esac

Expected output: no
Got output: yes

The above needs to be tested in a non-interactive shell (i.e. a script)
due to the "!". Other shells I've tested (bash, dash, ksh, pdksh, mksh)
behave as expected.

I confirmed the bug in zsh 4.3.11, zsh 5.0.2 and zsh 5.0.7-dev-2.

Thanks,

- Martijn