时间段的匹配:    ((0[0-9]|1[0-9]|2[0-3])\:(0[0-9]|[1-5][0-9])\_(0[0-9]|1[0-9]|2[0-3])\:(0[0-9]|[1-5][0-9]))

$time = '02:00_08:00';    //早上2点到8点

$patten = "((0[0-9]|1[0-9]|2[0-3])\:(0[0-9]|[1-5][0-9])\_(0[0-9]|1[0-9]|2[0-3])\:(0[0-9]|[1-5][0-9]))";

if(preg_match($patten, $time)){

        echo 'ok';

}