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

Question

Building an NSCharacter set to restrict a UITextField for entering user names. I want the user to be able to also enter an underscore (so [A-Za-z0-9_]) but alphanumericCharacter

Answer 1

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