After further experimentation, I suspect that "[^]" is simply not
a valid regular expression.  For instance, grep(1) gives the
following behavior:

    $ echo "^" | grep "[^]"
    grep: brackets ([ ]) not balanced

This suggests that the broken behavior is within
`regexp-opt-charset`.  I've attached a patch for that function.

Here are some test cases which reveal the behavior of the unpatched
and patched versions of the function (the only difference is the
handling of the "[^]" case):

    ;; Pre-patch
    (regexp-opt-charset (list ?^))          ; "[^]"
    (regexp-opt-charset (list ?^ ?a))       ; "[a^]"
    (regexp-opt-charset (list ?^ ?-))       ; "[-^]"
    (regexp-opt-charset (list ?^ ?\]))      ; "[]^]"
    (regexp-opt-charset (list ?^ ?- ?\]))   ; "[]^-]"

    ;; Post-patch
    (regexp-opt-charset (list ?^))          ; "\\^"
    (regexp-opt-charset (list ?^ ?a))       ; "[a^]"
    (regexp-opt-charset (list ?^ ?-))       ; "[-^]"
    (regexp-opt-charset (list ?^ ?\]))      ; "[]^]"
    (regexp-opt-charset (list ?^ ?- ?\]))   ; "[]^-]"

--
Cameron Desautels <camdez@gmail.com>