title: What does preceding a string literal with "r" mean?
body: <p>I first saw it used in building regular expressions across multiple lines as a method argument to <code>re.compile()</code>, so I assumed that <code>r</code> stands for RegEx.</p>
  
  <p>For example:</p>
  
  <pre><code>regex = re.compile(
      r'^[A-Z]'
      r'[A-Z0-9-]'
      r'[A-Z]$', re.IGNORECASE
  )
  </code></pre>
  
  <p>So what does <code>r</code> mean in this case? Why do we need it?</p>