验证邮箱地址方法

验证邮箱地址是否合法：

- (BOOL)isAvailableEmail:(NSString*)emailString{
    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:emailString]; 
}