problem with Adding a url pattern

from django.shortcuts import render

from .models import Article, Writer


def article_list(request):
    articles = Article.objects.all()
    return render(request, 'articles/article_list.html', {'articles': articles})


def writer_detail(request, pk):
    writer = Writer.objects.get(pk=pk)
    return render(request, 'articles/writer_detail.html', {'writer': writer})

def article_detail(request, pk):
  article = Article.objects.get(pk=pk)
  return render(request, 'articles/article_detail.html', {'article': article})

from django.conf.urls import url

from . import views

urlpatterns = [
    url(r'writer/(?P<pk>\d+)/$', views.writer_detail),
    url(r'', views.article_list),
    url(r'article/(?P<pk>\d+)/$', views.article_detail),
]