django正则表达式错误：没有什么可重复的

Question

我通常理解消息error: nothing to repeat表示某些正则表达式存在问题。但是我一直无法找出错误的位置。以下是我的代码中的正则表达式：

# ROOT URL

from django.conf import settings
from django.conf.urls import patterns, include, url

# Uncomment the next two lines to enable the admin:
from django.contrib import admin
admin.autodiscover()

urlpatterns = patterns('',
                       url(r'^rango/', include('rango.urls')),
                       url(r'^admin/', include(admin.site.urls)),
                       )


if settings.DEBUG:
    urlpatterns += patterns(
        'django.views.static',
        (r'media/(?P<path>.*)',
         'serve',
         {'document_root': settings.MEDIA_ROOT}), )


# RANGO APPLICATION URL

from django.conf.urls import patterns, url
from rango import views

urlpatterns = patterns('',
                       url(r'^$', views.index, name='index'),
                       url(r'^about/$', views.about, name='about'),
                       url(r'^add_category/$', views.add_category,
                           name='add_category'),
                       url(r'^category/(?P<category_name_url>\w+)/$',
                           views.category, name='category'),)

由于

编辑：添加了追溯

Traceback (most recent call last):
  File "/home/user/.virtualenvs/tango_with_django/local/lib/python2.7/site-packages/django/core/management/base.py", line 222, in run_from_argv
    self.execute(*args, **options.__dict__)
  File "/home/user/.virtualenvs/tango_with_django/local/lib/python2.7/site-packages/django/core/management/base.py", line 250, in execute
    translation.activate('en-us')
  File "/home/user/.virtualenvs/tango_with_django/local/lib/python2.7/site-packages/django/utils/translation/__init__.py", line 90, in activate
    return _trans.activate(language)
  File "/home/user/.virtualenvs/tango_with_django/local/lib/python2.7/site-packages/django/utils/translation/trans_real.py", line 183, in activate
    _active.value = translation(language)
  File "/home/user/.virtualenvs/tango_with_django/local/lib/python2.7/site-packages/django/utils/translation/trans_real.py", line 172, in translation
    default_translation = _fetch(settings.LANGUAGE_CODE)
  File "/home/user/.virtualenvs/tango_with_django/local/lib/python2.7/site-packages/django/utils/translation/trans_real.py", line 154, in _fetch
    app = import_module(appname)
  File "/home/user/.virtualenvs/tango_with_django/local/lib/python2.7/site-packages/django/utils/importlib.py", line 35, in import_module
    __import__(name)
  File "/home/user/.virtualenvs/tango_with_django/local/lib/python2.7/site-packages/django/contrib/admin/__init__.py", line 6, in <module>
    from django.contrib.admin.sites import AdminSite, site
  File "/home/user/.virtualenvs/tango_with_django/local/lib/python2.7/site-packages/django/contrib/admin/sites.py", line 4, in <module>
    from django.contrib.admin.forms import AdminAuthenticationForm
  File "/home/user/.virtualenvs/tango_with_django/local/lib/python2.7/site-packages/django/contrib/admin/forms.py", line 6, in <module>
    from django.contrib.auth.forms import AuthenticationForm
  File "/home/user/.virtualenvs/tango_with_django/local/lib/python2.7/site-packages/django/contrib/auth/forms.py", line 14, in <module>
    from django.contrib.auth.models import User
  File "/home/user/.virtualenvs/tango_with_django/local/lib/python2.7/site-packages/django/contrib/auth/models.py", line 18, in <module>
    from django.contrib.auth.hashers import (
  File "/home/user/.virtualenvs/tango_with_django/local/lib/python2.7/site-packages/django/contrib/auth/hashers.py", line 9, in <module>
    from django.test.signals import setting_changed
  File "/home/user/.virtualenvs/tango_with_django/local/lib/python2.7/site-packages/django/test/__init__.py", line 6, in <module>
    from django.test.testcases import (TestCase, TransactionTestCase,
  File "/home/user/.virtualenvs/tango_with_django/local/lib/python2.7/site-packages/django/test/testcases.py", line 35, in <module>
    from django.test import _doctest as doctest
  File "/home/user/.virtualenvs/tango_with_django/local/lib/python2.7/site-packages/django/test/_doctest.py", line 534, in <module>
    class DocTestParser:
  File "/home/user/.virtualenvs/tango_with_django/local/lib/python2.7/site-packages/django/test/_doctest.py", line 554, in DocTestParser
    ''', re.MULTILINE | re.VERBOSE)
  File "/home/user/.virtualenvs/tango_with_django/lib/python2.7/re.py", line 190, in compile
    return _compile(pattern, flags)
  File "/home/user/.virtualenvs/tango_with_django/lib/python2.7/re.py", line 242, in _compile
    raise error, v # invalid expression
error: nothing to repeat