<VirtualHost *:80>
    ServerAlias example.com
    RedirectMatch permanent ^/(.*) http://www.example.com/$1
</VirtualHost>

RewriteCond %{HTTP_HOST} ^!example.com$ [NC]
RewriteRule ^(.*)$ http://www.example.com/$1 [R=301,L]

This starts with the HTTP_HOST variable, which contains just the domain name portion of the incoming URL (example.com). Assuming the domain name does not contain a www. and matches your domain name exactly, then the RewriteRule comes into play. The pattern ^(.*)$ will match everything in the REQUEST_URI, which is the resource requested in the HTTP request (foo/blah/index.html). It stores this in a back reference, which is then used to rewrite the URL with the new domain name (one that starts with www).

[NC]表示不区分大小写的模式匹配，[R=301]表示使用代码301进行外部重定向(资源永久移动)，[L]停止所有进一步重写，并立即重定向。

使用重写引擎是解决这个问题的重量级方法。这里有一个更简单的解决方案:

<VirtualHost *:80>
    ServerName example.com
    Redirect permanent / http://www.example.com/
</VirtualHost>

<VirtualHost *:80>
    ServerName www.example.com
    # real server configuration
</VirtualHost>

然后，您将有另一个<VirtualHost>节，其中ServerName为www.example.com，用于您的真实服务器配置。当使用Redirect指令时，Apache会自动保留/之后的所有内容，这是一个常见的误解，为什么这个方法不起作用(实际上它可以)。

如果你有多个域名(可选)

-如果所有这些域名都使用https(因为他们应该)

-如果你想让所有的域名都使用www。domainName

这将避免doble重定向(http://non www到http://www，然后到https://www)

<VirtualHost *:80>
RewriteEngine On
RewriteCond %{HTTP_HOST} ^(?:www\.)?(.+)$ [NC]
RewriteRule ^(.*)$ https://www.%1$1 [R=301,L]
</VirtualHost>

And

<VirtualHost *:443>
RewriteEngine On
RewriteCond %{HTTP_HOST} !^www\. [NC]
RewriteRule ^(.*)$ https://www.%{HTTP_HOST}$1 [R=301,L]
</VirtualHost>

您应该将重定向代码301更改为最方便的代码

http://example.com/subdir/?lold=13666 => http://www.example.com/subdir/?lold=13666

RewriteEngine On
RewriteCond %{HTTP_HOST} !^www\. [NC]
RewriteRule ^(.*)$ http://www.%{HTTP_HOST}%{REQUEST_URI} [R=301,L]

重定向代码的非www => www和相反的www =>非www。在.htaccess文件中没有硬编码域和方案。因此，源域名和http/https版本将被保留。

Apache 2.4及更新版本

www => www:

RewriteEngine On
RewriteCond %{HTTP_HOST} !^www\. [NC]
RewriteRule ^ %{REQUEST_SCHEME}://www.%{HTTP_HOST}%{REQUEST_URI} [R=301,L]

WWW =>非WWW:

RewriteEngine On
RewriteCond %{HTTP_HOST} ^www\.(.*)$ [NC]
RewriteRule ^ %{REQUEST_SCHEME}://%1%{REQUEST_URI} [R=301,L]

注意:在Apache 2.2上无效，%{REQUEST_SCHEME}不可用。为了与Apache 2.2兼容，请使用下面的代码或将%{REQUEST_SCHEME}替换为固定的http/https。

Apache 2.2及更新版本

www => www:

RewriteEngine On

RewriteCond %{HTTPS} off
RewriteCond %{HTTP_HOST} !^www\. [NC]
RewriteRule ^ http://www.%{HTTP_HOST}%{REQUEST_URI} [R=301,L]

RewriteCond %{HTTPS} on
RewriteCond %{HTTP_HOST} !^www\. [NC]
RewriteRule ^ https://www.%{HTTP_HOST}%{REQUEST_URI} [R=301,L]

．.． 或者更短的版本……

RewriteEngine On
RewriteCond %{HTTP_HOST} !^www\. [NC]
RewriteCond %{HTTPS}s ^on(s)|offs
RewriteRule ^ http%1://www.%{HTTP_HOST}%{REQUEST_URI} [R=301,L]

WWW =>非WWW:

RewriteEngine On

RewriteCond %{HTTPS} off
RewriteCond %{HTTP_HOST} ^www\.(.*)$ [NC]
RewriteRule ^ http://%1%{REQUEST_URI} [R=301,L]

RewriteCond %{HTTPS} on
RewriteCond %{HTTP_HOST} ^www\.(.*)$ [NC]
RewriteRule ^ https://%1%{REQUEST_URI} [R=301,L]

．.． 更短的版本不可能，因为%N仅可从上次RewriteCond…