title: pandas: identifying if a first n characters are letter followed by numbers
body: <p>I have a dataframe with column which contains multiple reference values. I am trying to filter a ceratin group of references which follow this format:</p>
  <p><strong>ABCD12345678</strong></p>
  <p>Basically the first 4 characters are letters followed by 8 numbers.</p>
  <p>I tried:</p>
  <pre><code>df_new=df[df['col'].str.match('[a-zA-Z]', na = False)]
  </code></pre>
  <p>and</p>
  <pre><code>bew_2=df[df['col'].str.slice(0,4).str.contains('[a2-3]', na = False)]
  </code></pre>
  <p>But neither worked. It would be great if someone could guide me through this.</p>