Test UITextField text string to only contain alphanumeric characters

Question

Im trying to complete form validation in Swift and cant find a way of testing for only Alphanumeric characters in a UITextField.text.

Ive found NSCharacterSet to help test if at least 1 letter has been entered (so far):

@IBOutlet weak var username: UITextField!
let letters = NSCharacterSet.letterCharacterSet()

//Check username contains a letter
   if (username.text!.rangeOfCharacterFromSet(letters) == nil) {
        getAlert("Error", message: "Username must contain at least 1 letter")
    }

Now i just need a way to validate that only numbers, letters (maybe even underscores and dashes) to be entered. Loads of stuff out there for Obj-C but I need a SWIFT solution please.

Thank you in advance.

Answer 1

Check if the inversion of your accepted set is present:

if username.text!.rangeOfCharacterFromSet(letters.invertedSet) != nil {
    print("invalid")
}

letters should probably be alphanumericCharacterSet() if you want to include numbers as well.

If you want to accept underscores or more chars, you will probably have to create a character set by your own. But the inversion logic will stay the same.

Answer 2

Though it's late to answer, but this answer might be useful to someone.

This is simple and worked like a charm for me.

Swift 3:

func textField(_ textField: UITextField, shouldChangeCharactersIn range: NSRange, replacementString string: String) -> Bool {
    /// 1. replacementString is NOT empty means we are entering text or pasting text: perform the logic
    /// 2. replacementString is empty means we are deleting text: return true
    if string.characters.count > 0 {
        var allowedCharacters = CharacterSet.alphanumerics

        /// add characters which we need to be allowed
        allowedCharacters.insert(charactersIn: " -") // "white space & hyphen" 

        let unwantedStr = string.trimmingCharacters(in: allowedCharacters)
        return unwantedStr.characters.count == 0
    }

    return true
}

Note: This will work for pasting strings into the text field as well. Pasted string will not be displayed in text field if it contains any unwanted characters.