title: URL grammar rules
body: <p>I made a pastebin site where each entry gets a random string. For example</p>
  
  <pre><code>example.com/ds34
  example.com/sdf-2zA
  example.com/234+_2
  </code></pre>
  
  <p>My question is, what is the grammar rule for these strings?
  Can that start with anything? which characters are/aren't allowed?</p>