title: Python regex - r prefix
body: <p>Can anyone explain why example 1 below works, when the <code>r</code> prefix is not used?
  I thought the <code>r</code> prefix must be used whenever escape sequences are used.
  Example 2 and example 3 demonstrate this.</p>
  
  <pre><code># example 1
  import re
  print (re.sub('\s+', ' ', 'hello     there      there'))
  # prints 'hello there there' - not expected as r prefix is not used
  
  # example 2
  import re
  print (re.sub(r'(\b\w+)(\s+\1\b)+', r'\1', 'hello     there      there'))
  # prints 'hello     there' - as expected as r prefix is used
  
  # example 3
  import re
  print (re.sub('(\b\w+)(\s+\1\b)+', '\1', 'hello     there      there'))
  # prints 'hello     there      there' - as expected as r prefix is not used
  </code></pre>