如何将非www重定向到我的域中的www

I'm having a problem I just want to redirect the mydomain.com to www.mydomain.com, but the solutions I've found make my oder subdomains like admin.mydomain.com redirect to www.admin.mydomain.com. I have an Ubuntu 14.04 with Apache and working with Laravel (PHP).

I just want this to happen when is the mydomain.com/.... to www.mydomain.com/... but no when there is something else at the beginning like admin.mydomain.com

I leave here the solution I found but that has this little problem I just explained.

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

You can specify the domain you want to redirect:

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

Here is the rule that I use to force the use of www.

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

This should be placed only inside of the VirtualHost that has the top level definition of the domain

ServerName exmaple.com

Each other subdomain should have their own Virtual Host definition.

# Redirect anything except admin.yourdomain.com, admin1.yourdomain.com
RewriteCond %{HTTP_HOST} !^(admin|admin1)\.yourdomain\.com$ [NC]
# Redirect to www.yourdomain.com, without changing the URI
RewriteRule ^(.*)$ http://www.yourdomain.com/$1 [L,R=302]

For multiple subdomains this will work

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