PHP:用户名、密码和电子邮件的简单Regex

我正在尝试创建一个用于用户名、密码和电子邮件验证的简单regex检查器。

要求如下：

用户名：

- Only letters and numbers (a-z, A-z, 0-9). 
- No spaces, linebreaks, tabs or special characters.
- At least 5 characters in length.
- No more than 20 characters in length.

密码：

- At least 8 digits in length.
- no more than 40 characters in length.
- Must contain at least 1 special character or number.

电子邮件：

- Must follow the format "value@value.com".

这就是我目前所拥有的：

用户名-正在工作

$username = $enteredUsername;
if(preg_match('/^[a-zA-Z0-9]{5,30}+$/', $username) == false ) {
echo "the username is invalid";
}

密码：尚未工作：

$password = $enteredPassword;
if(preg_match('/^[a-zA-Z0-9]+['w0-9]{8,}+$/', $password) == false ) {
echo "the password is invalid";
}

电子邮件-正在工作

$email = $enetered email.
if (filter_var($email, FILTER_VALIDATE_EMAIL) == false) {
    echo "the email is invalid";
} 

电子邮件和用户名有效，但我似乎无法找到正确的密码正则表达式。