Fix re_match bug.

git-svn-id: http://svn.boost.org/svn/boost/trunk@83582 b8fc166d-592f-0410-95f2-cb63ce0dd405
I completely misread the definition of rfind.

git-svn-id: http://svn.boost.org/svn/boost/trunk@83583 b8fc166d-592f-0410-95f2-cb63ce0dd405