title: Replacing some characters in a string with another character
body: <p>I have a string like <code>AxxBCyyyDEFzzLMN</code> and I want to replace all the occurrences of <code>x</code>, <code>y</code>, and <code>z</code> with <code>_</code>.</p>
  <p>How can I achieve this?</p>
  <p>I know that <code>echo &quot;$string&quot; | tr 'x' '_' | tr 'y' '_'</code> would work, but I want to do that in one go, without using pipes.</p>