pattern matching - Why can't I replace I with HoldPattern@I?

Just try this sample:

I /. HoldPattern@I -> a
(* I *)

Why does it fail to replace I with a? I've checked // Hold // FullForm but found nothing useful.

Answer

@xzczd has always struck me as a very mature person, and this question hasn't really changed that opinion :-)

The answer is because I evaluates internally as Complex[0,1], so

I /. HoldPattern[Complex[0, _]] -> a

would work, or even

I /. HoldPattern[Complex[0, 1]] -> a

and in a more practical example,

FourierTransform[f[tt], tt, I ww] /. 
 HoldPattern@FourierTransform[a_, t_, Complex[0, 1] w_] :> 

  myfourier[a, t, J w]
(* myfourier[f[tt], tt, J ww] *)

Comments

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