title: how to fill with spaces an inserted expression to a given length during substitution in vim regex
body: <p>I want to implement behavior similar to <code>setw()</code> and <code>setfill()</code> in c ++. For example i want to replace expression:</p>
  <pre><code>15.8
  </code></pre>
  <p>with:</p>
  <pre><code>_______15,8
  </code></pre>
  <p>in text and:</p>
  <pre><code>10000
  </code></pre>
  <p>with:</p>
  <pre><code>_10000 
  </code></pre>