title: How to check if a string is inside another string (but not on the edge)?
body: <p>Let's say I have this string: <code>abc/def</code>. How would I check if there is a slash inside the string but not on the edge? For example, a slash at the very end like <code>abcdef/</code> or the very start like <code>/abcdef</code> is not allowed.</p>
  
  <p>In this case, <code>String#includes</code> doesn't work since that includes all possible locations. How do I achieve this?</p>