title: grep with regex
body: <p>I have two quick question with regex with grep.</p>
  
  <ol>
  <li><p>what does <code>\?</code> mean? I didn't find the explanation for question mark (<code>?</code>)</p></li>
  <li><p>How can I achieve <code>or</code>? for example, the phone number xxx-xxx-xxxx but the first part could be (xxx)</p></li>
  </ol>
  
  <p>For example, there might be a digit or not between two letters, such as <code>a1b</code> and <code>ab</code>, how to do that?</p>
  
  <p>Thanks!</p>