title: Number of keys that match a string
body: <p>I have the following hash:</p>
  
  <pre><code>{
    "day_mon"=&gt;"on", "day_tue"=&gt;"on", "day_wed"=&gt;"on", "day_thu"=&gt;"on",
    "day_fri"=&gt;"on", "day_sat"=&gt;"on", "day_sun"=&gt;"on"
  }
  </code></pre>
  
  <p>How can I count the number of keys in the hash that match the string <code>"day"</code>?</p>