I run the example:
c = a - b; e^(a - b)
This gives me e^(a-b). How can I do to let it output e^c
e^(a - b) // FullSimplify[#, c == Defer[c]] &
(e^(a - b)) /. a -> c + b
e^(a - b) // FullSimplify[#, c == Defer[c]] &? – kglr May 25 '21 at 10:34(e^(a - b)) /. a -> c + b. Have fun! – Alexei Boulbitch May 25 '21 at 11:23