title: Grep everything between two special character patterns
body: <p>I am having trouble figuring out how to grep the characters between two special character patterns.
  Specifically I need to grep everything between the first __ (double underscore) and the second __ I could use sed to convert them to spaces then cut the column, but that seems a bit unnecessary and messy. </p>
  
  <p>My data looks something like this....</p>
  
  <pre><code>364__spc__spc__1334
  608__aac(3)-Ik__aac(3)-Ik__1927
  13__tet(S)__tet(S)_1__1102
  </code></pre>
  
  <p>The result I need is... </p>
  
  <pre><code>spc
  aac(3)-Ik
  tet(S)
  </code></pre>
  
  <p>Thanks in advance to anyone with suggestions. </p>