title: Extract substring from column
body: <p>I'm trying to extract the hour from the entire data+time column df['Start Time']. Is there any easy way to do that using str.extract()? Any support is appreciated!!</p>
  
  <pre><code>    Start Time             
  2019-01-14 07:42:00    
  2019-01-14 05:29:00    
  2019-01-14 08:01:00    
  2019-01-18 04:20:00  
  2019-01-14 05:30:00   
         ... 
  2018-04-09 10:15:33   
  2018-12-14 06:36:46     
  2018-04-11 07:51:24    
  </code></pre>
  
  <p>I've tried:</p>
  
  <pre><code>df['Start Hour'] = df['Start Time'].str.split(' ', 1)
  </code></pre>
  
  <p>Output</p>
  
  <pre><code>    Start Time             
  2019-01-14   
  2019-01-14  
  2019-01-14    
  2019-01-18  
  2019-01-14  
         ... 
  2018-04-09    
  2018-12-14    
  2018-04-11 
  </code></pre>
  
  <p>Desired Output</p>
  
  <pre><code>    Start Time             
  07
  05 
  08  
  04
  05
     ... 
  10
  06  
  07
  </code></pre>