title: Getting numbers from a string with grep
body: <p>I came with another simple question...</p>
  
  <p>I got a string with a substring in the format xx:xx:xx where the x's are numbers. I want to extract that substring including the ":" symbol, so my output would be "xx:xx:xx".</p>
  
  <p>I think it can be done with a grep -Eo [0-9], but im not sure of the syntax... Any help?</p>