title: extract all email addresses from some .txt documents using ruby
body: <p>I have to extract all email addresses from some .txt documents. These emails may have these formats:</p>
  
  <ol>
  <li><code>a@abc.com</code></li>
  <li><code>{a, b, c}@abc.edu</code></li>
  <li>some other formats including some <code>@</code> signs.</li>
  </ol>
  
  <p>I choose ruby for my first language to write this program, but i don't know how to write the regex. Would someone help me? Thank you!</p>