银行卡号格式化

在输入银行卡号过程中，每隔4位插入一个"-"，类似"1332-2131-2313-1231-212"这种的
做法：

1. 设置textField的代理
2. 在以下代理方法中加入如下方法：

- (BOOL)textField:(UITextField *)textField shouldChangeCharactersInRange:(NSRange)range replacementString:(NSString *)string
{
    NSString *text = [textField text];
    NSCharacterSet *characterSet = [NSCharacterSet characterSetWithCharactersInString:@"0123456789\b"];
    string = [string stringByReplacingOccurrencesOfString:@"-" withString:@""];
    if ([string rangeOfCharacterFromSet:[characterSet invertedSet]].location != NSNotFound) {
        return NO;
    }
    
    text = [text stringByReplacingCharactersInRange:range withString:string];
    text = [text stringByReplacingOccurrencesOfString:@"-" withString:@""];
    
    NSString *newString = @"";
    while (text.length > 0) {
        NSString *subString = [text substringToIndex:MIN(text.length, 4)];
        newString = [newString stringByAppendingString:subString];
        if (subString.length == 4) {
            newString = [newString stringByAppendingString:@"-"];
        }
        text = [text substringFromIndex:MIN(text.length, 4)];
    }
    
    newString = [newString stringByTrimmingCharactersInSet:[characterSet invertedSet]];
    
    if (newString.length >= 24) {
        return NO;
    }
    
    [textField setText:newString];
    
    return NO;
}

3. 得到不包含"-"的卡号

[self.textField.text stringByReplacingOccurrencesOfString:@"-" withString:@""]

OK!