title: Golang matches characters to get substrings
body: <p><code>https://&lt;some strings&gt;appid=c5f1b8ee7da1&amp;source=&lt;some strings&gt;&amp;path=&lt;some strings&gt;</code></p>
  <p>I want to match <code>appid=</code> and <code>&amp;</code> to get <code>c5f1b8ee7da1</code></p>