Rewrite rule works perfect, but I don't want to rewrite www

Question

I have this rewrite rule which rewrite demo.mydomain.com to mydomain.com/myform.aspx?url=demo.

 <rewrite>
 <rules>
 <rule name="domain redirect" stopProcessing="true">
  <match url="^(.*)$" />
  <conditions trackAllCaptures="true">
<add input="{HTTP_HOST}" pattern="^(.+)\.mydomain\.com$" />
  </conditions>
   <action type="Rewrite" url="{REQUEST_URI}/myform.aspx?url={C:1}" appendQueryString="true" />
 </rule>
</rules>
</rewrite>

My problem is that http://www.mydomain.com is rewrited too. How can I exclude www in my rewrite rule?

Answer 1

 <add input="{HTTP_HOST}" pattern="^[^(www)](.+)\.mydomain\.com$" />