title: What do the braces () in URL regular expression represent?
body: <p>For example, in</p>
  
  <pre><code>r'^articles/(\d{4})/$', 'news.views.year_archive'
  </code></pre>
  
  <p>I understand all regexes except (\d{4}). Four digits but why the braces? </p>
  
  <p>(python/django example)</p>
  
  <p>another example:</p>
  
  <pre><code>r'^articles/(\d{4})/(\d{2})/(\d+)/$', 'news.views.article_detail'
  </code></pre>