title: Grep Pattern matching- underline
body: <p>I've not been able to find anything online to help so hoping someone may have an idea.</p>
  
  <p>What does an underline in an expression mean when using grep? </p>
  
  <p>For example: [_a-zA-Z0-9]</p>
  
  <p>Could someone help to explain the purpose here?</p>