# How to extract the argument from an in-built function

I would like to extract the argument from :

DiracDelta[-10 + x + ξ]


You can use Part to access the parts of any normal expression.

DiracDelta[-10 + x + ξ][[1]]
(* -10 + x + ξ *)


In this case you can also use First since you want the first part.

Another common need is to pull out the arguments of a function when that function is buried in a larger expression. For this you can use Cases

Cases[
{w''[z] + k^2 w[z] == DiracDelta[z], w[0] == 0, w'[0] == 1},
DiracDelta[arg_] :> arg, Infinity]
(* {z} *)


DiracDelta[-10 + x + \[Xi]] /. DiracDelta -> Identity
(**)


More generally(also works for several arguments)

 Apply[List ,DiracDelta[-10 + x + \[Xi]] ]

DiracDelta[-10 + x + \[Xi]] /. DiracDelta -> " "


or (as @JasonB wrote):

First@DiracDelta[-10 + x + \[Xi]]