Substitute symbolic sub-expression in expression

I have an equation

eq = Tnext - 2*Tn + Tprev == - sigma*Tinf^4*h^2 - alpha*Tinf*h^2 + sigma*Tn^4*h^2 + alpha*Tn*h^2

in which I want to replace the sub-expression

sigma*Tinf^4*h^2 - alpha*Tinf*h^2

with

C = sigma*Tinf^4*h^2 - alpha*Tinf*h^2

such that it becomes

eq = Tnext - 2*Tn + Tprev == - C + sigma*Tn^4*h^2 + alpha*Tn*h^2

I have tried the following:

eq_to_sub = sigma*Tinf^4*h^2 - alpha*Tinf*h^2;

syms C
eq = subs(eq, eq_to_sub, C)

subsx = @(u,w0,w0str) feval(symengine,'subsex',u,[char(w0) '=' w0str]);
eq = subsx(eq, eq_to_sub, 'C')

which doesnt change the equation at all. Weirdly, both methods work if the sub-expression (to substitute) is limited to

eq_to_sub = sigma*Tinf^4;

resulting in

Tnext - 2*Tn + Tprev == Tn*h^2*alpha - C*h^2 + Tn^4*h^2*sigma + (-h^2)*Tinf*alpha

[Valeri Aronov]

Note that the monomials in C have different signs, but the same monomials in eq have the same sign. C is not found in eq because of it.

Answer 1

[Jyotsna Talluri]

Enclose the subexpresssion in braces as it contains more than one term.

eq = Tnext - 2*Tn + Tprev == - (sigma*Tinf^4*h^2 + alpha*Tinf*h^2) + sigma*Tn^4*h^2 + alpha*Tn*h^2;
C = sigma*Tinf^4*h^2 + alpha*Tinf*h^2;
q = feval(symengine,'subsex',eq,[char(C) '=C']);

[Martin Androvich]

That's tricky, haha! But it works, so thanks a bunch 😊

[Martin Androvich]

But what if the expression is generated from some other function, e.g. simplify(), where I cannot simply manually enclose it in braces?