Collect the terms of a polynomial

p = x ^ 2 E ^ (2x) + 2x E ^ (2x) + x E ^ x

E^x*x + 2*E^(2*x)*x + E^(2*x)*x^2

Collect[p, x]

E^x + 2*E^(2*x) + E^(2*x)*x

Collect[(1 + a + x) ^ 4, x]

1 + 4*a + 6*a^2 + 4*a^3 + a^4 + x^4 + (4 + 12*a + 12*a^2 + 4*a^3)*x + (6 + 12*a + 6*a^2)*x^2 + (4 + 4*a)*x^3

Collect[(1 + a + x) ^ 4, x, Simplify]

(1 + a)^4 + x^4 + 4*(1 + a)^3*x + 6*(1 + a)^2*x^2 + 4*(1 + a)*x^3

Collect[(x + y + z + 1) ^ 4, {x, y}]

1 + x*(4 + 4*y^3 + 12*z + 12*z^2 + 4*z^3 + y^2*(12 + 12*z) + y*(12 + 24*z + 12*z^2)) + x^2*(6 + 6*y^2 + 12*z + 6*z^2 + y*(12 + 12*z)) + x^3*(4 + 4*y + 4*z) + x^4 + y*(4 + 12*z + 12*z^2 + 4*z^3) + y^2*(6 + 12*z + 6*z^2) + y^3*(4 + 4*z) + y^4 + 4*z + 6*z^2 + 4*z^3 + z^4