`RegexDispatch` is a little like `urldispatch.URLDispatch`, but allows
for more complex patterns and captured variables.  It works together
well with `caller.wsgify`.  First, we'll create an application with
`wsgify`::

    >>> from caller import wsgify
    >>> import calendar
    >>> @wsgify
    ... def calendar_year(req, year):
    ...     req.response.set_header('content-type', 'text/plain')
    ...     cal = calendar.calendar(int(year))
    ...     return str(cal)
    >>> @wsgify
    ... def calendar_month(req, year, month):
    ...     req.response.set_header('content-type', 'text/plain')
    ...     cal = calendar.month(int(year), int(month))
    ...     return str(cal)

Now we need to plug those applications together with some regular
expressions::

    >>> year_regex = r'^/cal/(?P<year>\d\d\d\d)/$'
    >>> month_regex = r'^/cal/(?P<year>\d\d\d\d)/(?P<month>\d\d)$'
    >>> from regexdispatch import RegexDispatch
    >>> dispatch = RegexDispatch([
    ...     (year_regex, calendar_year), 
    ...     (month_regex, calendar_month)])

Now, to test::

    >>> from testrunner import send_request
    >>> print send_request(dispatch, '/cal/1950/')
    200 OK
    Content-Type: text/plain
                                      1950
          January                   February                   March
    ...
    >>> print send_request(dispatch, '/cal/1950/02')
    200 OK
    Content-Type: text/plain
       February 1950
    Mo Tu We Th Fr Sa Su
           1  2  3  4  5
     6  7  8  9 10 11 12
    ...
    >>> print send_request(dispatch, '/')
    404 Not Found
    ...
    >>> print send_request(dispatch, '/cal/77/')
    404 Not Found
    ...