Allow only alphanumeric characters for a UITextField

Question

How would I go about allowing inputting only alphanumeric characters in an iOS UITextField?

Answer 1

Swift 3 version

Currently accepted answer approach:

func textField(_ textField: UITextField, shouldChangeCharactersIn range: NSRange, replacementString string: String) -> Bool {

    // Get invalid characters
    let invalidChars = NSCharacterSet.alphanumerics.inverted

    // Attempt to find the range of invalid characters in the input string. This returns an optional.
    let range = string.rangeOfCharacter(from: invalidChars)

    if range != nil {
        // We have found an invalid character, don't allow the change
        return false
    } else {
        // No invalid character, allow the change
        return true
    }
}

Another equally functional approach:

func textField(_ textField: UITextField, shouldChangeCharactersIn range: NSRange, replacementString string: String) -> Bool {

    // Get invalid characters
    let invalidChars = NSCharacterSet.alphanumerics.inverted

    // Make new string with invalid characters trimmed
    let newString = string.trimmingCharacters(in: invalidChars)

    if newString.characters.count < string.characters.count {
        // If there are less characters than we started with after trimming
        // this means there was an invalid character in the input. 
        // Don't let the change go through
        return false
    } else {
        // Otherwise let the change go through
        return true
    }

}