preg_match语法

Studying some code from a codeigniter tut, the following preg_match pattern has me baffled:

preg_match('/js$/', $include)

What is the purpose of the $ after the js?

Thanks for the always thoughtful replies!

-----Complete Code-----

<?php if ( ! defined('BASEPATH')) exit('No direct script access allowed');

/**
 * Layouts Class. PHP5 only.
 *
 */
class Layouts {

  // Will hold a CodeIgniter instance
  private $CI;

  // Will hold a title for the page, NULL by default
  private $title_for_layout = NULL;

  // The title separator, ' | ' by default
  private $title_separator = ' | ';

  public function __construct()
  {
    $this->CI =& get_instance();
  }

  public function set_title($title)
  {
    $this->title_for_layout = $title;
  }

  public function view($view_name, $params = array(), $layout = 'default')
  {
    // Handle the site's title. If NULL, don't add anything. If not, add a
    // separator and append the title.
    if ($this->title_for_layout !== NULL)
    {
      $separated_title_for_layout = $this->title_separator . $this->title_for_layout;
    }

    // Load the view's content, with the params passed
    $view_content = $this->CI->load->view($view_name, $params, TRUE);

    // Now load the layout, and pass the view we just rendered
    $this->CI->load->view('laytous/' . $layout, array(
      'content_for_layout' => $view_content,
      'title_for_layout' => $separated_title_for_layout
    ));
  }

  public function add_include($path, $prepend_base_url = TRUE)
  {
    if ($prepend_base_url)
    {
      $this->CI->load->helper('url'); // Load this just to be sure
      $this->file_includes[] = base_url() . $path;
    }
    else
    {
      $this->file_includes[] = $path;
    }

    return $this; // This allows chain-methods
  }

  public function print_includes()
  {
    // Initialize a string that will hold all includes
    $final_includes = '';

    foreach ($this->includes as $include)
    {
      // Check if it's a JS or a CSS file
      if (preg_match('/js$/', $include))
      {
        // It's a JS file
        $final_includes .= '<script type="text/javascript" src="' . $include . '"></script>';
      }
      elseif (preg_match('/css$/', $include))
      {
        // It's a CSS file
        $final_includes .= '<link href="' . $include . '" rel="stylesheet" type="text/css" />';
      }

      return $final_includes;
    }
  }
}

The dollar is an "end of string" anchor. The match will only succeed if "js" is at the end of the string.

Dollar sign means an end of line in regular expressions.