title: pep8 warning on regex string in Python, Eclipse
body: <p>Why is pep8 complaining on the next string in the code?</p>
  
  <pre><code>import re
  re.compile("\d{3}")
  </code></pre>
  
  <p>The warning I receive:</p>
  
  <pre><code>ID:W1401  Anomalous backslash in string: '\d'. String constant might be missing an r prefix.
  </code></pre>
  
  <p>Can you explain what is the meaning of the message? What do I need to change in the code so that the warning <em>W1401</em> is passed?</p>
  
  <p>The code passes the tests and runs as expected. Moreover <code>\d{3}</code> is a valid regex.</p>