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

Objective-C

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

Swift

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