Two recipes for extracting the last n words in a string

Method 1: rpartition

Again, I'm going to post the recipe up here, and go into depth below. If you want the last <n> words in any sentence, you can do a transform with:
rpartition(value, /(\s\S+){<n>}$/)[1]

Be sure that all your data is more than <n> words. If you have sentences that are exactly n words, you'll get a null.  You can either custom facet out that data with a :
length(value.split(" ")) ><n>

(what the length GREL does is described in this post.)

or use an if function:
 if (length(value.split(" ")) ><n>, rpartition(value,  /(\s\S+){<n>}$/)[1], value)


Method 2: Array arithmetic with split
 Since you can access any element in split() by using the expression split()[<index>], you can use length(value.split(" ")) to calculate the indexes you need.

To get the last <n> words in any sentence, do a transform with:
value.split(" ")[length(value.split(" "))-<n>] + " " +
value.split(" ")[length(value.split(" "))-<n-1>] + " " +
value.split(" ")[length(value.split(" "))-<n-2>] + " " +
value.split(" ")[length(value.split(" "))-<n-3>] +
etc. until <n-whatever> = 1

ex. If I want the last 4 words in any sentence, my transform GREL would be:
value.split(" ")[length(value.split(" "))-4] + " " +
value.split(" ")[length(value.split(" "))-3] + " " +
value.split(" ")[length(value.split(" "))-2] + " " +
value.split(" ")[length(value.split(" "))-1] 

Again, you will get unexpected results if your sentence is less than n words because negative indexes will wrap around. Either facet the short sentences away or use an if function:

 if (length(value.split(" ")) ><n>, value.split(" ")[length(value.split(" "))-<n>] + " " + value.split(" ")[length(value.split(" "))-<n-1>] + " " + value.split(" ")[length(value.split(" "))-<n-2>] + " " + value.split(" ")[length(value.split(" "))-<n-3>] + <etc.>, value)