title: Filter all rows that do not contain letters (alpha) in ´pandas´
body: <p>I am trying to filter a <code>pandas</code> dataframe using <code>regular expressions</code>.
  I want to delete those rows that do not contain any letters. For example:</p>
  
  <pre><code>Col A.
  50000
  $927848
  dog
  cat 583
  rabbit 444
  </code></pre>
  
  <p>My desired results is:</p>
  
  <pre><code>Col A.
  dog
  cat 583
  rabbit 444
  </code></pre>
  
  <p>I have been trying to solve this problem unsuccessful with <code>regex</code> and <code>pandas</code> filter options. See blow. I am specifically running into problems when I try to merge two conditions for the filter. How can I achieve this?</p>
  
  <p>Option 1:</p>
  
  <pre><code>df['Col A.'] = ~df['Col A.'].filter(regex='\d+')
  </code></pre>
  
  <p>Option 2</p>
  
  <pre><code>df['Col A.'] = df['Col A.'].filter(regex=\w+)
  </code></pre>
  
  <p>Option 3</p>
  
  <pre><code>from string import digits, letters
  df['Col A.'] = (df['Col A.'].filter(regex='|'.join(letters)))
  </code></pre>
  
  <p>OR </p>
  
  <pre><code>df['Col A.'] = ~(df['Col A.'].filter(regex='|'.join(digits)))
  </code></pre>
  
  <p>OR </p>
  
  <pre><code>df['Col A.'] = df[~(df['Col A.'].filter(regex='|'.join(digits))) &amp; (df['Col A.'].filter(regex='|'.join(letters)))]
  </code></pre>