这很简单!

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

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。

这是我自己网站的配置，工作起来很有魅力。

<IfModule mod_ssl.c>
  <VirtualHost *:443>
    ServerAdmin admin@domain.com

    ServerName www.domain.com
    ServerAlias domain.com

    DocumentRoot /var/www/html/domain

    <Directory /var/www/html/domain/>
      Options FollowSymLinks
      AllowOverride All
      Require all granted
    </Directory>

    # Redirect non-www to www
    RewriteEngine On

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

    ErrorLog ${APACHE_LOG_DIR}/error.log
    CustomLog ${APACHE_LOG_DIR}/access.log combined

    Include /etc/letsencrypt/options-ssl-apache.conf
    SSLCertificateFile /etc/letsencrypt/live/domain.com/fullchain.pem
    SSLCertificateKeyFile /etc/letsencrypt/live/domain.com/privkey.pem
  </VirtualHost>
</IfModule>

RewriteEngine上 @ HTTP_HOST} ^yourdomain.com [NC] RewriteRule ^(.*)$ http://www.yourdomain.com/$1 [L,R=301]

检查这个完美的工作

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

如果你正在使用Apache 2.4，不需要启用重写Apache模块，你可以使用这样的东西:

# non-www to www
<If "%{HTTP_HOST} = 'domain.com'">
  Redirect 301 "/" "http://www.domain.com/"
</If>