Using back-references in IIS rewrite rules

Parts of rules or conditions inputs can be captures in back-references. These can be then used to construct substitution URLs within rules actions or to construct input strings for rule conditions.

Back-references are generated in different ways, depending on which kind of pattern syntax is used for the rule. 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. For example, the pattern ([0-9]+)/([a-z]+)\.html will capture 07 and article in back-references from this requested URL: 07/article.html. 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. For example the pattern */*.html will capture contoso and test in back-references from this requested URL: contoso/test.html.

Usage of back-references is the same regardless of which pattern syntax was used to capture them. Back-references can be used in the following locations within rewrite rules:

• In condition input strings
• In rule actions, specifically:
  • url attribute of Rewrite and Redirect action
  • statusLine and responseLine of a CustomResponse action
• In a key parameter to the rewrite map

Back-references to condition patterns are identified by {C:N} where N is from 0 to 9. Back-references to rule patterns 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.

For example, in this pattern:

^(www\.)(.*)$

For the string: www.foo.com the back-references will be indexed as follows:

{C:0} - www.foo.com

{C:1} - www.

{C:2} - foo.com

Within a rule action, you can use the back-references to the rule pattern and to the last matched condition of that rule. Within a condition input string, you can use the back-references to the rule pattern and to the previously matched condition.

The following rule example demonstrates how back-references are created and referenced:

<rule name="Rewrite subdomain">
<match url=”^(.+)” > 
<conditions>
<add input="{HTTP_HOST}" type=”Pattern” pattern="^([^.]+)\.mysite\.com$"> 
</conditions>
<action type=”Rewrite” url="{C:1}/{R:1}" /> 
</rule>

Click here to read more about IIS rewrite rules