title: strange behaviour of regexp
body: <p>I have this str list </p>
  
  <pre><code>l=['Cu_3', 'P_2', 'O_8']
  </code></pre>
  
  <p>and this two regexp</p>
  
  <pre><code>import re
  sub = re.compile('_([0-9])')    
  sub2 = re.compile('._([0-9])')
  </code></pre>
  
  <p>sub doesn't match any elemetn of the list, while sub2 matches the last two but not the first one..
  I am probably missing something but I expected to capture the numbers after the underscore with both of them. What's the problem here?</p>