.htaccess rewrite url problem

[robert_gsfame]

I try to create a virtual subdomain by rewriting url in .htaccess
below is the code

<IfModule mod_rewrite.c>
Options +FollowSymLinks
Options +Indexes
RewriteEngine On
RewriteBase /
RewriteCond %{HTTP_HOST} !www.mydomain.co.id$ [NC]
RewriteCond %{HTTP_HOST} ^([a-z0-9-]+).mydomain.co.id [NC]
RewriteRule (.*) page.php?username=%1 [L]
</IfModule>
ErrorDocument 404 /page_error.html

the problem is that when i try this www.aaaaa.mydomain.co.id , i will then redirected to default web page cgi-sys... but once i try aaaaaa.mydomain.co.id then i will have this SERVER NOT FOUND

i really dont have anymore idea on how to solve this problem, hope that anyone could come up with a solution

thx in advance

[traq]

never tried it, but have you tried first rewriting all non-www urls with the www? So, in effect, if I typed sub.example.com, it would redirect to www.sub.example.com and then to the user page?

[robert_gsfame]

nope cause i dont have any idea on how to do it

i am new to .htaccess, could you point me out??

[traq]

I know a good htaccess tutorial - let me find it, and I'll try out some rules too.