Thanks for reporting that.  It's a bug in the glibc regular expression matcher, 
and as such will need a glibc fix.  I plan to propose a patch there.  In the 
meantime I fixed the Gnulib replacement for the matcher, here:

http://git.savannah.gnu.org/cgit/gnulib.git/commit/?id=5513b40999149090987a0341c018d05d3eea1272

and have installed the attached patches to GNU grep, so that the bug should be 
fixed if you configure grep using the --with-included-regex option.