title: What does <c-r>= means in Vim
body: <p>I came across a number of this syntax usages which I don't understand:</p>
  
  <p>The first is in :help mapping:</p>
  
  <pre><code>:map &lt;F2&gt; a&lt;C-R&gt;=strftime("%c")&lt;CR&gt;&lt;Esc&gt;
  </code></pre>
  
  <p>This sequence really does insert the value of strftime into buffer though I don't understand how. Changing  onto something different breaks it.</p>
  
  <p>Another one is at wiki <a href="http://vim.wikia.com/wiki/Improve_completion_popup_menu" rel="noreferrer">page</a> which describes how to make omnicompletion popup menu work well: </p>
  
  <pre><code>inoremap &lt;silent&gt; &lt;Esc&gt; &lt;C-r&gt;=pumvisible() ? "\&lt;C-y&gt;" : "\&lt;Esc&gt;"&lt;CR&gt;
  </code></pre>
  
  <p>The same thing here.</p>
  
  <p>Can anybody explain how this "<code>&lt;C-r&gt;=</code>" thing works?...</p>