title: What does "(?u)" do in a regex?
body: <p>I looked into how tokenization is implemented in scikit-learn and found this regex (<a href="https://github.com/scikit-learn/scikit-learn/blob/c957249/sklearn/feature_extraction/text.py#L1215" rel="noreferrer">source</a>):</p>
  
  <pre><code>token_pattern = r"(?u)\b\w\w+\b"
  </code></pre>
  
  <p>The regex is pretty straightforward but I have never seen the <code>(?u)</code> part before. Can someone explain me what this part is doing? </p>