BackReferences, R1, R2 in URL Redirect rule questions

Question

I tried to look for more info on back reference. But the ones i found aren't very clear. So I am really confused. I have this in a web.config

  <system.webServer>
<rewrite>
  <rules>
    <clear />

    <rule name="CTVNews Articles" stopProcessing="true">
      <match url="(.*?)CTVNews(/?)(.*)" />
      <action type="Redirect" url="http://www.ctvnews.ca{R:2}{R:3}" appendQueryString="true" redirectType="Permanent" />
    </rule>

so, I know that is the rule with regular expression.That is easy to understand, but what about url="http://www.ctvnews.ca/generic{R:2}{R:3}. Can someone please kindly explain what is R2 and R3? and why we have R2, R3, instead of R1 and R2?

All help is greatly appreciate it.

Answer 1

In the XML snippet you provided, the regular expression has three "capturing groups" - that is, use of parentheses ("(" and ")"). These "capture" whatever is inside them, and can then be referred to by number. There are numbered left-to-right, starting from 1.

Then in the <action> tag above, the {R:2} parts are referring to what the groups in the regular expression captured. There are three such groups, but the <action> is only using the second and third. The first one is being captured but is not being used.