title: Regex force length of specific regex
body: <p>I'm using R and need a regex for</p>
  
  <blockquote>
    <p>a block of <strong>N</strong> characters starting with zero or more
    whitespaces and continuing with one or more digits afterwards</p>
  </blockquote>
  
  <p>For <strong>N</strong> = 9 here are</p>
  
  <p><strong>examples of valid strings</strong></p>
  
  <ul>
  <li><code>123456789</code></li>
  <li><code>kfasdf  3456789asdf</code></li>
  <li><code>a        1</code></li>
  </ul>
  
  <p>and <strong>examples of invalid strings</strong></p>
  
  <ul>
  <li><code>12345 789</code></li>
  <li><code>1       9</code></li>
  <li><code>a     678a</code></li>
  </ul>