How to use regex to replace domain name in php

<?php 
    //网站的url
    $url = &#39;http://www.baidu.com/index.php&#39;;
    //正则表达式
    $reg = &#39;/(http):\/\/([^\/]+)/i&#39;;
    preg_match($reg, $url,$res);
    /**  $res的结果
            array (size=3)
=> string &#39;http://www.baidu.com&#39; (length=20)
=> string &#39;http&#39; (length=4)
=> string &#39;www.baidu.com&#39; (length=13)
    */
    //要替换的位置替换成什么
    $url1 = &#39;www.jingdong.com&#39;;
 
    /**
    Example #1 使用后向引用紧跟数值原文
    */
    echo preg_replace($reg, &#39;http://&#39;.$url1, $url);
 
        
    /**
    Example #2 preg_replace()中使用基于索引的数组
    */
    $patterns[0] = &#39;/&#39;.$res[2].&#39;/&#39;;
    $replacements[0] = $url1;
    echo preg_replace($patterns, $replacements, $url);
    //结果为 http://www.jingdong.com/index.php
 
 
 
 ?>