Allow only alphanumeric characters for a UITextField

Use the UITextFieldDelegate method -textField:shouldChangeCharactersInRange:replacementString: with an NSCharacterSet containing the inverse of the characters you want to allow. For example:

// in -init, -initWithNibName:bundle:, or similarNSCharacterSet *blockedCharacters = [[[NSCharacterSet alphanumericCharacterSet] invertedSet] retain];- (BOOL)textField:(UITextField *)field shouldChangeCharactersInRange:(NSRange)range replacementString:(NSString *)characters{    return ([characters rangeOfCharacterFromSet:blockedCharacters].location == NSNotFound);}// in -dealloc[blockedCharacters release];

Note that you’ll need to declare that your class implements the protocol (i.e. @interface MyClass : SomeSuperclass <UITextFieldDelegate>) and set the text field’s delegate to the instance of your class.

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    }}

This is how I do it:

// Define some constants:#define ALPHA                   @"ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz"#define NUMERIC                 @"1234567890"#define ALPHA_NUMERIC           ALPHA NUMERIC// Make sure you are the text fields 'delegate', then this will get called before text gets changed.- (BOOL)textField:(UITextField *)textField shouldChangeCharactersInRange:(NSRange)range replacementString:(NSString *)string {    // This will be the character set of characters I do not want in my text field.  Then if the replacement string contains any of the characters, return NO so that the text does not change.    NSCharacterSet *unacceptedInput = nil;    // I have 4 types of textFields in my view, each one needs to deny a specific set of characters:    if (textField == emailField) {        //  Validating an email address doesnt work 100% yet, but I am working on it....  The rest work great!        if ([[textField.text componentsSeparatedByString:@"@"] count] > 1) {            unacceptedInput = [[NSCharacterSet characterSetWithCharactersInString:[ALPHA_NUMERIC stringByAppendingString:@".-"]] invertedSet];        } else {            unacceptedInput = [[NSCharacterSet characterSetWithCharactersInString:[ALPHA_NUMERIC stringByAppendingString:@".!#$%&'*+-/=?^_`{|}~@"]] invertedSet];        }    } else if (textField == phoneField) {        unacceptedInput = [[NSCharacterSet characterSetWithCharactersInString:NUMERIC] invertedSet];    } else if (textField == fNameField || textField == lNameField) {        unacceptedInput = [[NSCharacterSet characterSetWithCharactersInString:ALPHA] invertedSet];    } else {        unacceptedInput = [[NSCharacterSet illegalCharacterSet] invertedSet];    }    // If there are any characters that I do not want in the text field, return NO.    return ([[string componentsSeparatedByCharactersInSet:unacceptedInput] count] <= 1);}

Check out the UITextFieldDelegate Reference too.