* Modify org-ts-regexpr0 to satisfy Karl Voit :-)

This is an attempt to allow timestamps without a day-of-week.

Modifying complicated regexps is no fun: I had to resort to re-builder
to figure out what was going on. The problem was that the first " *" which
is supposed to eat all the spaces between the "2012-03-05" part and
the "Mon" part, would, in the absence of the "Mon" part, eat the space
before the time spec "08:00". But the regexp for the time part explicitly
requires that space (see the second sp in the explanatory line below).
So I tried making the DOW part optional by appending a ?, but that didn't
do the trick. re-builder showed the space problem: there was no space left
to match if DOW is absent, so I changed the literal space " " to 0 or more
spaces " *" and that did the trick (with some caveats noted below).

In the code block below, tslist is a list of test cases. The code
block sets org-ts-regexp0 first to its original value, then to the
changed value (the first one is there just for comparison between the
two). I've marked the changes (of which, the second is the significant
one). We map the function org-parse-time-string over the test
cases and get a list of results. The results look reasonable, but the
new regexp allows funny-looking input like the second and fourth test
case.

One additional caveat is that org-odt uses org-ts-regexp0, so changing
it without checking org-odt's use is fraught with peril, but that's what
I've done: I have no idea whether it causes problems in org-odt.

#+BEGIN_SRC elisp
  ;                           year_______    -   month______    -   day________    sp   DOW____________       sp  hour_________    :   minutes____
  (setq org-ts-regexp0 "\\(\\([0-9]\\{4\\}\\)-\\([0-9]\\{2\\}\\)-\\([0-9]\\{2\\}\\) *\\([^]+0-9>\r\n -]*\\)\\( \\([0-9]\\{1,2\\}\\):\\([0-9]\\{2\\}\\)\\)?\\)")
  ; changed                                                                                                v    v
  (setq org-ts-regexp0 "\\(\\([0-9]\\{4\\}\\)-\\([0-9]\\{2\\}\\)-\\([0-9]\\{2\\}\\) *\\([^]+0-9>\r\n -]*\\)?\\( *\\([0-9]\\{1,2\\}\\):\\([0-9]\\{2\\}\\)\\)?\\)")
  (setq tslist '(
          "<2012-03-05 Mon 08:00>"
          "<2012-03-05 Mon08:00>"
          "<2012-03-05 Mon 8:00>"
          "<2012-03-05 Mon8:00>"
          "<2012-03-05 Mon     8:00>"
          "<2012-03-05    Mon     8:00>"
          "<2012-03-05 Mon >"
          "<2012-03-05 Mon>"
          "<2012-03-05 08:00>"
          "<2012-03-05 8:00>"
          "<2012-03-05       8:00>"
          "<2012-03-05>"))
  
  (pp (mapcar 'org-parse-time-string tslist))
  
  
#+END_SRC

#+RESULTS:
#+begin_example
((0 0 8 5 3 2012 nil nil nil)
 (0 0 8 5 3 2012 nil nil nil)
 (0 0 8 5 3 2012 nil nil nil)
 (0 0 8 5 3 2012 nil nil nil)
 (0 0 8 5 3 2012 nil nil nil)
 (0 0 8 5 3 2012 nil nil nil)
 (0 0 0 5 3 2012 nil nil nil)
 (0 0 0 5 3 2012 nil nil nil)
 (0 0 8 5 3 2012 nil nil nil)
 (0 0 8 5 3 2012 nil nil nil)
 (0 0 8 5 3 2012 nil nil nil)
 (0 0 0 5 3 2012 nil nil nil))
#+end_example