title: Deleting a rows with invalid date in pandas column
body: <p>I have a data frame as follows:</p>
  
  <pre><code>df:
  date       value
  20171231   500
  20161231   321.25
  20111512   234
  20151215   900.7
  19900101   6252.1
  19791131   123
  ...
  </code></pre>
  
  <p>where date column is an integer and in the format YYYYMMDD. Before converting into <strong>pd.to_datetime</strong> I want to filter this column with valid dates between 1980 to present day.</p>
  
  <p>What would be an efficient way to perform this operation? using regexp or any other pandas method available?</p>