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Event attributes are one of the most important sources of data for computation in a rule because they convey information about the context of the event. In Chapter 3, you saw how eventexes can use expressions on event attributes as filters for which events match a given primitive eventex. Event attributes can also be used in KRL expressions. To avoid conflict with user-declared variables names, event attributes are namespaced using the keyword event. (Prepending the namespace name and a colon to a variable forms namespaces in KRL.)

Assume that a rule has been selected based on a pageview event with an event attribute url. The following KRL expression could be used to extract the domain name from the url attribute:
AnchorOLE_LINK3OLE_LINK3 AnchorOLE_LINK4OLE_LINK4 

event:url.extract(#http://(\[^/?]+)#)

Event attributes can be sent to an event in several different ways. If one is submitting an event using a GET or POST request, the GET or POST arguments can be accessed using the following syntax:

event:attr("argument_name")

Event attributes can also be sent when raising an explicit event using the with keyword. The following KRL would raise an event with event attributes:

raise explicit event testEventAttributes for a1x100

   with foo = "bar";

This attribute could be used in the following rule:

rule testEventAttributes {

   select when explicit testEventAttributes

      pre {

         foo = event:attr("foo"); // Equals bar at this point

      }

      noop()

}