title: What's the difference between () and [] in regular expression patterns?
body: <p>What is the difference between encasing part of a regular expression in () (parentheses) and doing it in [] (square brackets)?  </p>
  
  <p>How does this:</p>
  
  <pre><code>[a-z0-9]
  </code></pre>
  
  <p>differ from this:</p>
  
  <pre><code>(a-z0-9)
  </code></pre>
  
  <p>?</p>