import Foundation

let foo = "_"

let bar = CharacterSet.alphanumerics.union(CharacterSet(charactersIn: foo))

let baz = foo.rangeOfCharacter(from: bar.inverted)

let qux = (baz == nil)

print("baz: \(baz as Any)")

// On macOS this prints `true` and on Linux this prints `false`.
print("qux: \(qux)")