title: how to filter letter and number in one column with pandas
body: <p>I want to filter number less than 3 <strong>including ".":</strong></p>
  
  <pre><code>import pandas as pd
  data = {"numbers":[1,2,".", 3,4, "."]}
  data
  {'numbers': [1, 2, '.', 3, 4, '.']}
  df=pd.DataFrame(data)
  df
  numbers
  0       1
  1       2
  2       .
  3       3
  4       4
  5       .
  </code></pre>
  
  <p>The result I want should like this:</p>
  
  <pre><code>numbers
  0       1
  1       2
  2       .
  5       .
  </code></pre>
  
  <p>Can someone help me?
  Thanks a lot.</p>