`ruby-forward-string' is a function used to skip past strings in Ruby
mode. Ruby supports using any non-alpha character following a "%" to
delimit a string.  All of the following are valid Ruby syntax:

    - %[foo bar]
    - %/foo bar/
    - %%foo bar%
    - %^foo bar^

Currently ruby-mode can handle all of these except the last one, which
yields the following:

    #     Debugger entered--Lisp error: (invalid-regexp "Unmatched [ or [^")

The root of the problem is the construction of the following invalid
regexp: "[^]" (which is an incomplete negated character class).

This bug manifests itself by breaking `ruby-parse-region' in any
buffer containing a caret-delimited string.  This breaks indentation,
movement commands, and even keeps Emacs from building an Imenu.

I've attached a patch which resolves the issue.  Note that this patch
builds on the test suite for `ruby-forward-string' that I included in
issue #16078.
--
Cameron Desautels <camdez@gmail.com>