title: Javascript regex with 20 chars, more than n numbers in a row
body: <p>I'm trying to put two regex's together and haven't figured it out.</p>
  
  <p>I've got the following to match only alphanumeric</p>
  
  <pre><code>^([a-zA-Z0-9_-]){1,20}$
  </code></pre>
  
  <p>and i've got the following to match any string with more than 7 numbers in a row</p>
  
  <pre><code>^\d{8,20}$
  </code></pre>
  
  <p>How do I put the two together to allow for only 20 alphanumeric characters, with more than 7 numbers in a row</p>