关于PHP正则匹配取出数据的有关问题

$exam = curl_init("http://exam.hhit.edu.cn/fgquery.do?status=lowquery&tsid=2012120348");
curl_setopt($exam, CURLOPT_RETURNTRANSFER, true); // 获取数据返回
curl_setopt($exam, CURLOPT_BINARYTRANSFER, true); // 在启用 CURLOPT_RETURNTRANSFER 时候将获取数据返回
$exam=curl_exec($exam);
        $exam = preg_replace("']*?>'si","",$exam);  
        $exam = preg_replace("']*?>'si","",$exam);   
        $exam = preg_replace("']*?>'si","",$exam);   
        $exam = str_replace("","{tr}",$exam);   
        $exam = str_replace("","{td}",$exam);   
        //去掉 HTML 标记    
        $exam = preg_replace("'<[/!]*?[^<>]*?>'si","",$exam);  
        //去掉空白字符     
        $exam = preg_replace("'([rn])[s]+'","",$exam);
        $exam = preg_replace('/ /',"",$exam);   
        $exam = str_replace(" ","",$exam);   
        $exam = str_replace(" ","",$exam); 
        $exam = explode('{tr}', $exam);
        array_pop($exam);       
        print_r($exam);
?>

[19] => 
编号{td}
课程{td}
日期{td}
时间{td}
班级{td}
考场{td}
任课教师{td}

[20] => 
1{td}
流体力学{td}
2014-11-0600:00:00.0
{td}
14:00-16:00{td}
土木122{td}
Ⅲ-209{td}
巩妮娜{td}

print_r(array_map(null, explode('{td}', $exam[19]), explode('{td}', $exam[20])));

Array
(
    [0] => Array
        (
            [0] => 编号
            [1] => 1
        )

    [1] => Array
        (
            [0] => 课程
            [1] => 流体力学
        )

    [2] => Array
        (
            [0] => 日期
            [1] => 2014-11-0600:00:00.0
        )

    [3] => Array
        (
            [0] => 时间
            [1] => 14:00-16:00
        )

    [4] => Array
        (
            [0] => 班级
            [1] => 土木122
        )

    [5] => Array
        (
            [0] => 考场
            [1] => Ⅲ-209
        )

    [6] => Array
        (
            [0] => 任课教师
            [1] => 巩妮娜
        )

    [7] => Array
        (
            [0] => 
            [1] => 
        )

)

print_r(array_combine(explode('{td}', $exam[19]), explode('{td}', $exam[20])));

Array
(
    [编号] => 1