It is recommended to look at Wolfram's notes of the NKS book (for the specific chapter). This will help you figure out the programming part, as there are numerous programming examples written by Stephen.

best

You should be more specific or a least give a snippet of code, since ReplaceAll replaces only one (and only the first replacement rule that matches) Try this and notice that the rule is used only once.

Range[5]/. {left___, x_, y_, right___} :> {left, {x, y}, right}

If you have two rules, only the first that matches is used, and only once (ReplaceAll is /. )

ReplaceAll[
 Range[5], {{left___, x_, y_, right___} :> {left, {x, y}, 
    right}, {left___, x_, y_, z_, right___} :> {left, {x, y, z}, 
    right}}]

If you would like to make all the replacements possible, than you need to use ReplaceList

ReplaceList[ Range[5], {left, x, y, right} :> {left, {x, y}, right}] or

ReplaceList[
 Range[5], {{left___, x_, y_, right___} :> {left, {x, y}, 
    right}, {left___, x_, y_, z_, right___} :> {left, {x, y, z}, 
    right}}]

So, it matters which replacement instructions is used (there are more than just the two I mentioned)