Something like ^(www\.)(.+)$ should work when matching the http_host, but it might be better to specify the domain. From what I know of IIS (not much) and what it says on the net, something like:
<rewrite>
<rules>
<rule name="Redirect www.xxx.com to xxx.com" patternSyntax="ECMAScript" stopProcessing="true">
<match url=".*" />
<conditions>
<add input="{HTTP_HOST}" pattern="^www\.domain\.com$" />
</conditions>
<action type="Redirect" url="http://domain.com/{R:0}" />
</rule>
</rules>
</rewrite>
Oh, you said for any domain. If you want to make sure it ends in .com it should be something like
^(www\.)(.+)(\.com)$ against HTTP_HOST
.. oh, if you do that you need to back reference, so try something like this:
<rewrite>
<rules>
<rule name="Redirect www.domain.com to domain.com" patternSyntax="ECMAScript" stopProcessing="true">
<match url=".*" />
<conditions>
<add input="{HTTP_HOST}" pattern="^www\.(.+)$" />
</conditions>
<action type="Redirect" url="http://{C:1}/{R:0}" redirectType="Permanent" />
</rule>
</rules>
</rewrite>
The c:1 is a back reference