Hi Babel

I've been working on an exporter and have come across some strange
behaviour regarding 'empty' source blocks with regard to tangling.

If an attempt is made to tangle a block containing two blank (no
white-space) lines, the block is exported as carriage returns (as I
would expect).

If a block contains one or no blank lines, the #+end_src tag appears to
be consumed by the block (along with subsequent text upto the next
#+end_src tag).

I tracked this down to the org-babel-src-block-regexp variable in ob.el
and have modified it to give what appears to be consistent behaviour.

I also found the following unresolved bug report which may be related...

http://www.mail-archive.com/emacs-orgmode@gnu.org/msg29992.html

I've included a simple test file, and resultant tangled files before and
after the patch to attempt to convey the problem.

NOTE: Since the patch itself modifies a defvar, and since I'm not clear
on how a patched defvar should be updated downstream, the patch includes a
setq statement to update the defvar org-babel-src-block-regexp for test
purposes. The patch cannot therefore be applied as is.