邮箱校验首尾不能符号

function bol() {
        var email = 'a-.a-c345.a@163-a-a-a.com';
        //var emailRule = /^[a-zA-Z0-9\._-]+@[a-zA-Z0-9_-]+(\.[a-zA-Z0-9_-]+)+$/;
    var emailRule = /^[a-zA-Z0-9]+([\._-]*[a-zA-Z0-9]+)*@[a-zA-Z0-9]+([\_-]*[a-zA-Z0-9]+)*(\.[a-zA-Z0-9_-]+)+$/;
        if (email == "") {
          alert('请输入邮箱');
            return false;
        } else if (!emailRule.test(email)) {
            /alert('邮箱格式不正确');
            return false;
        } else {
           alert('ok');
            return true;
        }
    }