Pattern recognition doesn't work as I would expect when trying to match products, for example

Cases[a[1] b[1] a[2] b[2], x___ a[k_] b[k_] -> k, All]


Cases[a[1] b[1] a[2] b[2], a[k_] b[k_] -> k, All]


But none of these two return {1,2} as I would expect. How to cure this problem ?



2 Answers 2


It works as expected. It matches the whole expression at once:

Cases[a[1] b[1] a[2] b[2], x___ a[k_] b[k_], All]
(* {a[1] a[2] b[1] b[2]} *)

If you want to get all possible matches, please look at ReplaceList, which:

attempts to transform the entire expression expr by applying a rule or list of rules in all possible ways, and returns a list of the results obtained.

ReplaceList[a[1] b[1] a[2] b[2], x___ a[k_] b[k_] :> k]
(* {1, 2} *)
  • $\begingroup$ Thanks a lot for your comment, it was really helpful! Also why does ReplaceList[a[1]^2 , x___ a[k_]^2 :> k] return {} instead of {1} ? $\endgroup$ Commented Mar 12 at 18:00
  • $\begingroup$ @AdrienMartina why are you using triple blank? Just use the regular blank, and make it optional: ReplaceList[a[1]^2 , x_. a[k_]^2 :> k] . But note that you can use plain Cases in this case. $\endgroup$
    – Domen
    Commented Mar 12 at 19:02
p = a[1] b[1] a[2] b[2];

SequenceCases[List @@ p, {a[k_], b[k_] ...} :> k]

{1, 2}


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