title: Parsing of a string with the length specified within the string
body: <p>Example data:</p>
  
  <blockquote>
    <p>029Extract this specific string. Do not capture anything else.</p>
  </blockquote>
  
  <p>In the example above, I would like to capture the first n characters immediately after the 3 digit entry which defines the value of n. I.E. the 29 characters "Extract this specific string."</p>
  
  <p>I can do this within a loop, but it is slow. I would like (if it is possible) to achieve this with a single regex statement instead, using some kind of backreference. Something like:</p>
  
  <pre><code>(\d{3})(.{\1})
  </code></pre>