title: Regex look ahead assertion
body: <p>I need a <code>regex</code> expert on this problem. It's linked to a SO question I've lost, where the data are the following:</p>
  
  <pre><code>x = c("IID:WE:G12D/V/A", "GH:SQ:p.R172W/G", "HH:WG:p.S122F/H")
  </code></pre>
  
  <p>I need to split each element of <code>x</code> to isolate the end part which can be consituted of <code>letter - slash - letter - .... slash - letter</code>.
  What I want is to obtain these two vectors as output:</p>
  
  <pre><code>o1 = c("IID:WE:G12", "GH:SQ:p.R172", "HH:WG:p.S122")
  o2 = c("D/V/A", "W/G", "F/H")
  </code></pre>
  
  <p>I have this solution for <code>o1</code>:</p>
  
  <pre><code>gsub('[A-Z]/.+','',x)
  #[1] "IID:WE:G12"   "GH:SQ:p.R172" "HH:WG:p.S122"
  </code></pre>
  
  <p>Good. For <code>o2</code>, I tried to use assertion and particularly look-ahead assertion:</p>
  
  <pre><code>gsub('.+(?=[A-Z]/.+)','',x, perl=T)
  #[1] "V/A" "W/G" "F/H"
  </code></pre>
  
  <p>But this is not the wanted result!</p>
  
  <p>Any idea what is going wrong with the second regex?</p>