Extract powers of exponential into a list

Question

I have a very complicated, long (thousands of terms) expression that boils down to a form of

expr = a1 Exp[b1] + a2 Exp[b2] + a3 Exp[b3]

Note that all a and b are huge symbolic expressions but the expression is expanded that way.

Now I would like to get a list with b1, b2, b3. All I can do is

Collect[expr, E^_]

but that does not give me a list.

Answer 1

You can use Cases:

Cases[expr, Exp[p_] :> p, All]

{b1, b2, b3}

Alternatively, you can use Exponent after changing the head of expr to List:

Exponent[List @@ expr, E]

{b1, b2, b3}