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Why BlankNullSequence rest___ is not matched with nothing (empty expression) ?

a^2 /. Times[rest___, x_^2] -> 2
(* a^2 *)

a^2 /. Plus[rest___, x_^2] :> 2
(* a^2 *)

Should not the answer be 2 in both cases ? How should be the pattern for that ?

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  • $\begingroup$ The rule will not match because $a^2$ head is Power, and therefore will be ignored. $\endgroup$
    – Anjan Kumar
    Commented Mar 13, 2017 at 12:47
  • $\begingroup$ @AnjanKumar, So what is wrong with the head Power ? Both a^2 and x_^2 have the same head Power. $\endgroup$
    – Mher
    Commented Mar 13, 2017 at 12:49

1 Answer 1

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You are asking to match a pattern that has a head of Times and your expression has a head of Power.

Use something like:

a^2 /. Times[n_., x_^2] -> 2

to do what you require. See the documentation for Default (n_.) and OneIdentity. OneIdentity is the attribute of Plus and Times which tells Mathematica that Times[x] is the same as x.

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