This looks like a case for the (often-overlooked) two-argument form of First:
list = {a + b, c + d, Psi*Zeta + Xi, 0};
First[#, #] & /@ list
{a, c, Xi, 0}
Second example from Nasser's answer with exceptions can be handled using:
listb = {a + b, c + d, Psi*Zeta + Xi, 0, Exp[x], Sin[x], x + Sin[x],
1 + Exp[x], x*Sin[x], Sin[x]^2, 1/x + 2, {1, 2, 3}};
listb /. $pat : Except[_Power | _List | _?(Length @ # == 1 &)] :> First[$pat, $pat]
This looks like a case for the (often-overlooked) two-argument form of First:
list = {a + b, c + d, Psi*Zeta + Xi, 0};
First[#, #] & /@ list
{a, c, Xi, 0}
Second example from Nasser's answer with exceptions can be handled using:
listb = {a + b, c + d, Psi*Zeta + Xi, 0, Exp[x], Sin[x], x + Sin[x],
1 + Exp[x], x*Sin[x], Sin[x]^2, 1/x + 2, {1, 2, 3}};
listb /. $pat : Except[_Power | _List | _?(Length @ # == 1 &)] :> First[$pat, $pat]
This looks like a case for the (often-overlooked) two-argument form of First:
list = {a + b, c + d, Psi*Zeta + Xi, 0};
First[#, #] & /@ list
{a, c, Xi, 0}
Second example from Nasser's answer with exceptions can be handled using:
listb = {a + b, c + d, Psi*Zeta + Xi, 0, Exp[x], Sin[x], x + Sin[x],
1 + Exp[x], x*Sin[x], Sin[x]^2, 1/x + 2, {1, 2, 3}};
listb /. $pat : Except[_Power | _List | _?(Length @ # == 1 &)] :> First[$pat, $pat]
This looks like a case for the (often-overlooked) two-argument form of First:
list = {a + b, c + d, Psi*Zeta + Xi, 0};
First[#, #] & /@ list
{a, c, Xi, 0}
Second example from Nasser's answer with exceptions can be handled using:
listb = {a + b, c + d, Psi*Zeta + Xi, 0, Exp[x], Sin[x], x + Sin[x],
1 + Exp[x], x*Sin[x], Sin[x]^2, 1/x + 2, {1, 2, 3}};
listb /. $pat : Except[_Power | _List | _?(Length @ # == 1 &)] :> First[$pat, $pat]