In this particular case you can do

 $p1:person (name=="Alice")
 $p3:person (name=="David", this after[0,1] $p1)
 not person (name != "Bob", this after[0,1] $p1 && before[0,1] $p3 )

A more general approach should be possible by monitoring states rather than event sequences, especially if you process events in real time.

1) Alice => state := A
2a) David => OK
2b) Bob  => state := AB
2c) Edward => state := _

And so on.
-W



On 20 July 2011 22:21, Edward Johnson <ejohnson@coderyte.com> wrote:
How do you do rules that are a sequence of events where some of them are optional?

Example:

Alert if DAVID enters a room after ALICE or after ALICE and BOB
If it were a string, this is roughly equivalant to the regex /ALICE( BOB)? DAVID/

So:
Alice then David => alert
Alice then Bob then David => alert
Alice then Charles then David => no alert

I could write this as two rules but as the rules get more complex it could by hard to manage the resulting combatorial explosion.  Here it is as two rules:

Rule 1:
 $p1:person (name=="Alice")
 $p2:person (name=="David", this after[0,1] $p1)

Rule 2:
 $p1:person (name=="Alice")
 $p2:person (name=="Bob", this after[0,1] $p1)
 $p3:person (name=="David", this after[0,1] $p2)
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