title: 'str' object is not callable in Django url pattern
body: <p>In my urls.py file, I have</p>
  
  <pre><code>urlpatterns = patterns('',
                         (r'^$',home_page),
                         #(r'^'+main_cagetory_url_string+'$','home_page'),
                         (r'^(?:cam_sanh|buoi_da_xanh|cam_da_xanh)$','home_page'),
                         (r'^admin/', include(admin.site.urls)),)
  </code></pre>
  
  <p>I want to use that pattern to access cam_sanh, buoi_da_xanh, cam_da_xanh page.
  But I receive error:</p>
  
  <pre><code>'str' object is not callable
  </code></pre>
  
  <p>How can I fix this bug?</p>