title: What this sequence '[[A-Z][a-z]]' means in regular expression in python?
body: <p>I was trying to search some patterns in regular expression in python. </p>
  
  <p>As we know if we use the pattern, <strong>'[A-Za-z]+'</strong> , it means find the sequence of  characters which either contains uppercase A to Z or lowercase a to z.
  So my single line of code (followed by answer) was,</p>
  
  <pre><code>&gt;&gt;&gt; re.findall('[A-Za-z]+', 'This is my area!')
  ['This', 'is', 'my', 'area']
  </code></pre>
  
  <p>When I tried some another pattern <strong>'[[A-Z][a-z]]+'</strong> it returned an empty list, single line of code is as follow,</p>
  
  <pre><code>&gt;&gt;&gt; re.findall('[[A-Z][a-z]]+', 'This is my area!')
  []
  </code></pre>
  
  <p>So, when I use two sequence inside a character set, what pattern is getting created ?
  Please guide. </p>