NSCharacterSet: How do I add "_" to alphanumericCharacterSet text restriction?

Question

Objective-C

NSMutableCharacterSet *_alnum = [NSMutableCharacterSet characterSetWithCharactersInString:@"_"];
[_alnum formUnionWithCharacterSet:[NSCharacterSet alphanumericCharacterSet]];

Swift

let _alnum = NSMutableCharacterSet(charactersIn: "_")
_alnum.formUnion(with: .alphanumerics)

Another way would have been to make it mutable and add it.

Objective-C

NSMutableCharacterSet *characterSet = [NSMutableCharacterSet alphanumericCharacterSet];
[characterSet addCharactersInString:@"_"];

Swift

let characterSet = NSMutableCharacterSet.alphanumeric()
characterSet.addCharacters(in: "_")

You could verify it has been added (in a Playground) with:

characterSet.characterIsMember(UInt16(Character("^").unicodeScalars.first!.value)) // false
characterSet.characterIsMember(UInt16(Character("_").unicodeScalars.first!.value)) // true -- YAY!
characterSet.characterIsMember(UInt16(Character("`").unicodeScalars.first!.value)) // false