title: Use perl=TRUE regex in dplyr select
body: <p>How can I select cols using <code>perl = TRUE</code> like regex.</p>
  
  <pre><code>data.frame(baa=0,boo=0,boa=0,lol=0,bAa=0) %&gt;% dplyr::select(matches("(?i)b(?!a)"))
  </code></pre>
  
  <blockquote>
    <p>Error in grep(needle, haystack, ...) :    invalid regular expression
    '(?i)b(?!a)', reason 'Invalid regexp'</p>
  </blockquote>
  
  <p>regex is indeed valid.</p>
  
  <pre><code>grep("(?i)b(?!a)",c("baa","boo","boa","lol","bAa"),perl=T)
  
  &gt; [1] 2 3
  </code></pre>
  
  <p>Is there a shortcut function/way?</p>