title: In sed, how to represent "alphanumeric or _ or -"
body: <p>Tried</p>
  
  <pre><code>[a-zA-Z0-9-_]
  [a-zA-Z0-0\-\_]
  [[[:alnum:]]-_]
  ...
  </code></pre>
  
  <p>What is the correct way to represent that in regular expression?</p>
  
  <p>It seems like [a-zA-Z0-9-] works for alphanumeric or dash. But when I add another underscore, it breaks.</p>