IIS URL Rewrite {R:N} clarification

Question

I've not been able to understand the purpose of {R:N}. Could anyone please clarify when to use
{R:0} vs. {R:1}

usage example:

<action type="Redirect" url="http://www.{HTTP_HOST}/{R:0}" /> 

I've seen ScottGu using {R:1}

http://weblogs.asp.net/scottgu/archive/2010/04/20/tip-trick-fix-common-seo-problems-using-the-url-rewrite-extension.aspx

Whereas, below has {R:0}

http://weblogs.asp.net/owscott/archive/2009/11/27/iis-url-rewrite-rewriting-non-www-to-www.aspx

Had a look at the IIS link below but could not quite digest the definition below:

Back-references to condition patterns are identified by {C:N} where N is from 0 to 9; back-references to rule pattern are identified by {R:N} where N is from 0 to 9. Note that for both types of back-references, {R:0} and {C:0}, will contain the matched string

Answer 1

As per the documentation:

When an ECMAScript pattern syntax is used, a back-reference can be created by putting parenthesis around the part of the pattern that must capture the back-reference.

So taking the example that follows in the documentation:

^(www\.)(.*)$ 

And using the input string www.foo.com in the conditions, you will have:

{C:0} - www.foo.com {C:1} - www. {C:2} - foo.com 

To make it simple:

• {R:x} is used as back reference from the rule pattern (<match url="...">).
• {C:x} is used as back reference from the condition pattern (<conditions><add input="{HTTP_HOST}" pattern="..."></conditions>)
• The 0 reference contains the whole input string
• The 1 reference will contain the first part of the string matching the pattern in the first parenthesis (), the 2 reference the second one, etc...up to the reference number 9

Note:

When "Wildcard" pattern syntax is used, the back-references are always created when an asterisk symbol (*) is used in the pattern. No back-references are created when "?" is used in the pattern.

http://www.iis.net/learn/extensions/url-rewrite-module/url-rewrite-module-configuration-reference#Using_back-references_in_rewrite_rules