A question from RFC 3403

[sandoche BALAKRICHENAN]

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:" ?

The RFC says

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?






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