Fixed #6170 -- Ensured that a useful exception is raised when a regex is invalid in the URLConf.

Thanks to abrahamson.j for the report, to guettli for initial work on the patch, and to David Gouldin for the new patch and test.

django/core/urlresolvers.py

@@ -160,10 +160,16 @@ class LocaleRegexProvider(object):
         language_code = get_language()
         if language_code not in self._regex_dict:
             if isinstance(self._regex, basestring):
-                compiled_regex = re.compile(self._regex, re.UNICODE)
+                regex = self._regex
             else:
                 regex = force_unicode(self._regex)
+            try:
                 compiled_regex = re.compile(regex, re.UNICODE)
+            except re.error, e:
+                raise ImproperlyConfigured(
+                    u'"%s" is not a valid regular expression: %s' %
+                    (regex, unicode(e)))
+
             self._regex_dict[language_code] = compiled_regex
         return self._regex_dict[language_code]