title: grep: repetition-operator operand invalid
body: <p>I have this regular express <code>(?&lt;=heads\/)(.*?)(?=\n)</code> and you can see it working here
  <a href="http://regexr.com?347dm" rel="noreferrer">http://regexr.com?347dm</a></p>
  
  <p>I need this regex to work in the grep command but I'm getting this error.</p>
  
  <pre><code>$ grep -Eio '(?&lt;=heads\/)(.*?)(?=\n)' text.txt 
  grep: repetition-operator operand invalid
  </code></pre>
  
  <p>It works great in ack but I dont have ack on the machine I need to run this on.</p>
  
  <pre><code>ack text.txt -o --match '(?&lt;=heads\/)(.*?)(?=\n)'
  </code></pre>
  
  <p>text.txt</p>
  
  <pre><code>74f3649af36984e1b784e46502fe318e91d29570    HEAD
  06d4463ab47a6246e6bd94dc3b9267d59fc16c2e    refs/heads/ARC
  0597e13c22b6397a1b260951f9d064f668b26f08    refs/heads/LocationAge
  e7e1ed942d15efb387c878b9d0335b37560c8807    refs/heads/feature/311-312-breaking-banner-updates
  d0b2632b465702d840a358d0b192198ae505011c    refs/heads/gulf-news
  509173eafc6792739787787de0d23b0c804d4593    refs/heads/jbb-new-applicationdidfinishlaunching
  1e7b03ce75b1a7ba47ff4fb5128bc0bf43a7393b    refs/heads/locationdebug
  74f3649af36984e1b784e46502fe318e91d29570    refs/heads/master
  5d2ede384325877c24db7ba1ba0338dc7b7f84fb    refs/heads/mixed-media
  3f3b6a81dd3baea8744aec6b95c2fe4aaeb20ea3    refs/heads/post-onezero
  4198a43aab2dfe72d7ae9e9e53fbb401fc9dac1f    refs/heads/whitelabel
  76741013b3b2200de29f53800d51dfd6dc7bac5e    refs/tags/r10
  fc53b1a05dad3072614fb397a228819a67615b82    refs/tags/r10^{}
  afdcfd970c9387f6fda0390ef781c2776aa666c3    refs/tags/r11
  </code></pre>