php过滤文本字符串中的url链接地址

$str    ="http://www.httproot.com我的网址";
$str1    ="支付宝的网址是HTTPS://www.alipay.com你记住了吗";//https支持，大写支持
$str2    ="http://news.baidu.com百度新闻,过滤url链接";


function removeLinks($str){
    if(empty($str)){return '';}
    $str    =preg_replace('/(http)(.)*([a-z0-9\-\.\_])+/i','',$str);
    $str    =preg_replace('/(www)(.)*([a-z0-9\-\.\_])+/i','',$str);
    return    $str;
}

echo removeLinks($str).'';
echo removeLinks($str1).'';
echo removeLinks($str2).'';