title: about backtrack of regex
body: <p><a href="https://i.stack.imgur.com/vmjAT.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/vmjAT.png" alt="enter image description here"></a></p>
  
  <p>Hi all. I have a question.</p>
  
  <p>In no.6 level, why backtrack to 'Welco'?</p>
  
  <p>I think backtrack to 'Welcom' is correct in no.6 level.</p>
  
  <p>Already been used 'Welcom' in no.4 level? </p>
  
  <p>English writing is very difficult.:)</p>