title: Match number at the start of string
body: <p>I am working on a data science project in which i need to extract number of hours from column 'content duration' which has instances like '2.5 hours'. 
  I tried running the following code :</p>
  
  <pre><code>df['num_hours'] = df['content_duration'].str.extract('(^\d*)')
  </code></pre>
  
  <p>But output of this returns "2" which means it excludes everything after "." 
  also, some instances of 'content duration' are like '39 hours' so extract only 39</p>
  
  <p>So, how to extract everything except string in the column 'content duration' ? </p>