title: regex - append after first match
body: <p>Say I have the following kind of file:</p>
  
  <pre><code>a a c
  b b c
  c c c
  d d c
  e e c
  a a c
  b b c
  c c c
  d d c
  e e c
  </code></pre>
  
  <p>How do I end up with:</p>
  
  <pre><code>a a c
  b b c
  c c c
  —————
  d d c
  e e c
  a a c
  b b c
  c c c
  d d c
  e e c
  </code></pre>
  
  <p>...without just adding the <code>em dashes</code> after the 3rd line (first line with <code>c c c</code>).</p>