title: grep egrep multiple-strings
body: <p>Suppose I have several strings: str1 and str2 and str3.</p>
  
  <ul>
  <li>How to find lines that have all the strings?</li>
  <li>How to find lines that can have any of them?</li>
  <li>And how to find lines that have str1 and either of str2 and str3 [<em>but not both?</em>]?</li>
  </ul>