Sometimes you don't know or want to paste the exact arguments to some mocked function, but you still want to assert that it was called in a certain manner, and you do know some things about the call arguments.

For example:

using Random: randstring
using SimpleMock: called_with, mock, SomethingThatIDontKnowWhatToName

f(x) = x * randstring()

mock(*) do star
    f("foo")
    @test called_with(star, "foo", SomethingThatIDontKnowWhatToName(x -> match(r"\s{8}", x) !== nothing)
end

Instead of comparing the observed argument, which in this case is the output of randstring(), to the expected argument by value, the function x -> match(r"\s{8}", x) !== nothing would run on the observed argument, and be considered a match if that outputted true.