Simple subdomain url rewriting

[deezerd]

Hi there
here is my problem:
I would like to set up a very simple rewriting subdomain rule that should look like this:
http://example.mywebsite.com => http://www.mywebsite.com/index.php?var=example

so I tried this rewrite rule but it doesn't work and I don't see what is wrong in it:
RewriteCond %{HTTP_HOST} ^{example.mywebsite.com} [NC]
RewriteRule (.*) http://www.mywebsite.com/index.php?var=$1 [R=301,L]

thanks in advance for your help