A question from RFC 3403

[SM]

At 07:22 26-05-2009, sandoche BALAKRICHENAN wrote:
>An example from RFC 3403
>
>The URN might look like this:
>
>     urn:cid:199606121851.1 at bar.example.com
>
>  This Application's First Well Known Rule is to extract the characters
>  between the first and second colon.  For this URN that would be
>  'cid'.  The Application also specifies that, in order to build a
>  Database-valid Key, the string 'urn.arpa' should be appended to the
>  result of the First Well Known Rule.  The result is 'cid.urn.arpa'.
>  Next, the client queries the DNS for NAPTR records for the domain-
>  name 'cid.urn.arpa'.  The result is a single record:
>
>cid.urn.arpa.   IN NAPTR 100   10   ""    ""
>"!^urn:cid:.+@([^\.]+\.)(.*)$!\2!i"
>
>My question is when the application has already converted 
>"urn:cid:199606121851.1 at bar.example.com" -> cid.urn.arpa.
>
>==> why does the regexp string again searches for  "urn:cid:" ?

Because it's not a terminal lookup.

>REGEXP - A <character-string> containing a substitution expression 
>that is applied to the original string
>
>==> Anyone have an idea why it always should be applied to the 
>original string?

The answer is in the paragraph that follows the one you quoted.

Regards,
-sm