Php 使用preg_match检查电子邮件域

我如何修改下面的preg_match函数以允许所有电子邮件地址以harvard.edu电子邮件地址结尾，而不是仅以@college.harvard.edu结尾

if(!preg_match("/@college\.harvard\.edu$/", $_POST["username"]))
    {
        apologize("You must provide a Harvard email to register.");
    }  

if(!preg_match("/@.*harvard\.edu$/", $_POST["username"]))
{
    apologize("You must provide a Harvard email address to register.");
}