Replace bunch of numbers by letter

Question

I want to do following:

ArcTan[1/Sqrt[-1 + (4 Sqrt[2] x^2)/(5 xc^2)]] /.4 Sqrt[2]/5->a^2

I expect to have

ArcTan[1/Sqrt[-1 + x^2/(a^2 xc^2)]]

Mathematica cannot recognize 4 Sqrt[2]/5 in expression.

Please provide a short but complete example, not just a fragment. Also explain what output you're expecting and what you actually got. — Szabolcs, Commented Feb 6, 2015 at 18:49
I was going to say what @Manuel --Moe-- G noted. I'll add that I for one believe this was a deplorable lexing/parsing decision, made by the wrong person, and retained ever since I have no idea why. The rule everywhere else is that a lexeme is as long as it can be. I will add that this bites me particularly hard because I often need to replace zeros (why that happens I don't know, but it does happen frequently). — Daniel Lichtblau, Commented Feb 7, 2015 at 22:37

Dr. belisarius · Accepted Answer · 2015-02-06 18:51:51Z

1

Sqrt[5/3] + 1 /. Sqrt[5/3] -> a
(* 1 + a *)

Tanh[Sqrt[5/3] x] /. Sqrt[5/3] ->  a
(* Tanh[a x] *)

Try starting a new Mathematica session

answered Feb 6, 2015 at 18:51

community wiki

$\begingroup$ sorry, I tried to keep it as simple as possible, but it does not cover my problem (see edit) $\endgroup$
– stinglikeabeer
Commented Feb 6, 2015 at 18:57
$\begingroup$ @belarus28 Copy this as is and execute ArcTan[1/Sqrt[-1 + (4 Sqrt[2] x^2)/(5 xc^2)]] /. 4 Sqrt[2]/5 -> a^2 $\endgroup$
– Dr. belisarius
Commented Feb 6, 2015 at 19:01
$\begingroup$ wow! it was not working, and now in the new session it works... magic. thanks. $\endgroup$
– stinglikeabeer
Commented Feb 6, 2015 at 19:12

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