IIS URL Rewrite Module Canonical Link Only If No Subdomain - regex

Question

I am using the IIS URL Rewrite module on a Windows 2012 server. I only want to add a canonical www rule if a subdomain is missing (or an exact match on 'example.com').
The problem is the ready-made Canonical rewrite rule IIS creates rewrites all traffic (*) for all subdomains to www.
<rule name="CanonicalHostNameRule1">
<match url="(.*)" />
<conditions>
<add input="{HTTP_HOST}" pattern="^www\.example\.com$" negate="true" />
</conditions>
<action type="Redirect" url="http://www.example.com/{R:1}" />
</rule>
The problem is that I have multiple subdomains / applications under one site (and therefore one web.config).
I want
example.com
to go to
www.example.com
but to leave these and any other subdomain alone:
secure.example.com
profile.example.com
Do I need a more specific match regex or pattern?
More specifically, the secure subdomain uses https and a better encryption certificate.

Answer 1

You're looking for everything that is not www.example.com. And that includes all other sub-domains.
Let's have a look at your condition:
<add input="{HTTP_HOST}" pattern="^www\.example\.com$" negate="true" />
If www.example.com comes in --> match = true --> negate --> false --> nothing is changed (as you want)
If example.com comes in --> match = false --> negate --> true --> URL gets redirected (as you want)
If foo.example.com comes in --> match = false --> negate --> true --> URL get redirected (as you don't want)
To fix this, try this configuration:
<rule name="Redirect to www" stopProcessing="true">
<match url="(.*)" />
<conditions trackAllCaptures="true">
<add input="{CACHE_URL}" pattern="^(.+)://" />
<add input="{HTTP_HOST}" pattern="^example\.com$" />
</conditions>
<action type="Redirect" url="{C:1}://www.example.com/{R:1}" />
</rule>
The C:1 part is for the protocol (http and https).
Found here.

Answer 2

This regex is better:
^(w{3}\.)?\w+\.com/
This regex will give you the option to get url with and without the www. and the / at the end, but still wont math a subdomain.