discourse/app/views/exceptions/not_found.html.erb

<% local_domain = "#{request.protocol}#{request.host_with_port}" %>

<p><%= t 'page_not_found.title' %></p>

<div class="row">
  <div class="span8">
    <h2><%= t 'page_not_found.popular_topics' %></h2>
    <% @top_viewed.each do |t| %>
      <%= link_to t.title, t.relative_url %><br/>
    <% end %>
    <br/>
    <a href="/latest" class="btn"><%= t 'page_not_found.see_more' %>&hellip;</a>
  </div>
  <div class="span8">
    <h2><%= t 'page_not_found.recent_topics' %></h2>
    <% @recent.each do |t| %>
      <%= link_to t.title, t.relative_url %><br/>
    <% end %>
    <br/>
    <a href="/latest" class="btn"><%= t 'page_not_found.see_more' %>&hellip;</a>
  </div>
</div>

<div class="row">
  <div class="span10" style='padding-top: 20px'>
    <h2><%= t 'page_not_found.search_title' %></h2>
    <p>
      <form action='https://google.com' id='google-search' onsubmit="return google_button_clicked()">
        <input type="text" id='user-query' value="<%= @slug %>">
        <input type='hidden' id='google-query' name="q">
        <button class="btn btn-primary"><%= t 'page_not_found.search_google' %></button>
      </form>
    </p>
  </div>
</div>

<script language="Javascript">
  function google_button_clicked(e) {
    var searchValue = document.getElementById('user-query').value;
    document.getElementById('google-query').value = 'site:<%= local_domain %> ' + searchValue;
    return true;
  }
</script>