title: Repetitive Sequence in Bash
body: <p>I'm stuck in something in my bash script, I have a string that composes of a repetitive sequence of <code>20</code>s, e.g. <code>202020</code>, there might be more or less <code>20</code>s, e.g. <code>2020</code> or <code>2020202020</code>, I want to create an if condition that if finds any strange number inside, e.g. <code>30</code> in <code>20203020</code>, gives an error.</p>
  
  <p>Any ideas?</p>