How to write a pattern that matches all functions

What is the correct way to match a function in an expression in the way that

Cases[Sin[x] Cos[x], _Sin | _Cos, {0, Infinity}]

matches Sin and Cos?

How can I match those functions without list all function names? The following code not only matches functions, but also List,Plus` and so on.

Cases[Sin[x] Cos[x], _?(MatchQ[Head@#, _Symbol] &), Infinity]

Should I express this question as matching a function not in a list such as {List, Plus, Times, ...}?

Answer

Cases[Sin[x] Cos[x], _@_, {0, Infinity}]
(* {Cos[x], Sin[x]} *)


Cases[Sin[x] Cos[x], h_@_ :> h, {0, Infinity}]
(* {Cos, Sin} *)

Note: watch out for expressions that "look like" functions with a single argument:

Cases[Sqrt[x ] Times[Sin[x], w], h_@_ :> h, {0, Infinity}]
(* {Sin} *)

because

Sqrt[x] // FullForm
(* Power[x,Rational[1,2]] *)

Comments

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Given a function f[x] , I would like to have a function leadingSeries that returns just the leading term in the series around x=0 . For example: leadingSeries[(1/x + 2)/(4 + 1/x^2 + x)] x and leadingSeries[(1/x + 2 + (1 - 1/x^3)/4)/(4 + x)] -(1/(16 x^3)) Is there such a function in Mathematica? Or maybe one can implement it efficiently? EDIT I finally went with the following implementation, based on Carl Woll 's answer: lds[ex_,x_]:=( (ex/.x->(x+O[x]^2))/.SeriesData[U_,Z_,L_List,Mi_,Ma_,De_]:>SeriesData[U,Z,{L[[1]]},Mi,Mi+1,De]//Quiet//Normal) The advantage is, that this one also properly works with functions whose leading term is a constant: lds[Exp[x],x] 1 Answer Update 1 Updated to eliminate SeriesData and to not return additional terms Perhaps you could use: leadingSeries[expr_, x_] := Normal[expr /. x->(x+O[x]^2) /. a_List :> Take[a, 1]] Then for your examples: leadingSeries[(1/x + 2)/(4 + 1/x^2 + x), x] leadingSeries[Exp[x], x] leadingSeries[(1/x + 2 + (1 - 1/x...

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