Misunderstanding posix-search-backward

[Peter S Galbraith]

I must be missing something obviously simple here...
Under emacs-20.7, I would think that (posix-search-backward "[0-9]+")
would match the longest possible string, but it stops after a
single character match:
 
(defun simple-test ()
  (interactive)
  (posix-search-backward "[0-9]+" nil t)
  (message "%s" (match-string 0)))

Position cursor at end of this line 0224234234 right here:

Running 'M-x simple-test' up here produces "4" instead of
"0224234234".

What am I missing?
Thanks,

-- 
Peter Galbraith, research scientist          <GalbraithP@dfo-mpo.gc.ca>
Maurice Lamontagne Institute, Department of Fisheries and Oceans Canada
P.O. Box 1000, Mont-Joli Qc, G5H 3Z4 Canada. 418-775-0852 FAX: 775-0546
    6623'rd GNU/Linux user at the Counter - http://counter.li.org/