Validating the email address in UITextField

NSString *emailReg = @"[A-Z0-9a-z._%+-]+@[A-Za-z0-9.-]+\\.[A-Za-z]{2,4}";
NSPredicate *emailTest = [NSPredicate predicateWithFormat:@"SELF MATCHES %@", emailReg];

if ([emailTest evaluateWithObject:yourtextFieldName.text] == NO) 
{

UIAlertView *alert = [[UIAlertView alloc] initWithTitle:@"enter the Valid Mail id" message:@"Please Enter Valid Email Address." delegate:nil cancelButtonTitle:@"okay" otherButtonTitles:nil];
[alert show];

}

Swift

 var emailReg: String = "[A-Z0-9a-z._%+-]+@[A-Za-z0-9.-]+\\.[A-Za-z]{2,4}"
var emailTest: NSPredicate = NSPredicate(format: "SELF MATCHES %@", emailReg)

if emailTest.evaluateWithObject(yourtextFieldName.text!) == false {
var alert: UIAlertView = UIAlertView(title: "enter the Valid Mail id", message: "Please Enter Valid Email Address.", delegate: nil, cancelButtonTitle: "okay", otherButtonTitles: "")
alert.show()
}

Swift3

let emailReg = "[A-Z0-9a-z._%+-]+@[A-Za-z0-9.-]+\\.[A-Za-z]{2,4}"
let emailTest = NSPredicate(format: "SELF MATCHES %@", emailReg)
if emailTest.evaluate(withObject: yourtextFieldName.text) == false {
 UIAlertView(title: "enter the Valid Mail id", message: "Please Enter Valid Email Address.", delegate: nil, cancelButtonTitle: "okay", otherButtonTitles: "").show()
 }

There are better ways to validate an email address.

- (BOOL)validateEmailWithString:(NSString*)email
{
    NSString *emailRegex = @"[A-Z0-9a-z._%+-]+@[A-Za-z0-9.-]+\\.[A-Za-z]{2,4}"; 
    NSPredicate *emailTest = [NSPredicate predicateWithFormat:@"SELF MATCHES %@", emailRegex]; 
    return [emailTest evaluateWithObject:email];
}