- URL pattern markers used in URL dispatch are permitted to specify a custom
  regex.  For example, the pattern ``/{foo:\d+}`` means to match ``/12345``
  (foo==12345 in the match dictionary) but not ``/abc``.  However, custom
  regexes in a pattern marker which used squiggly brackets did not work.  For
  example, ``/{foo:\d{4}}`` would fail to match ``/1234`` and
  ``/{foo:\d{1,2}}`` would fail to match ``/1`` or ``/11``.  One level of
  inner squiggly brackets is now recognized so that the prior two patterns
  given as examples now work.  See also
  https://github.com/Pylons/pyramid/issues/#issue/123.

Closes #123