Regular expression format with six numbers

I have this regular expression

 `NSString *expression = @"^([0-9]+)?(\\.?\\,?([0-9]{1,2})?)?$";`

 but i need a limit to 6 number before the dot or the comma and permit always two decimals.

Each caracters is checked with regular expression if it is valid i can write.

The problem is that i can write XXXXXXXX (8 numbers) but not six numbers or (XXXXXX.XX) (8 numbers and 2 decimals) for example with this expression

- (BOOL)textField:(UITextField *)textField shouldChangeCharactersInRange:(NSRange)range replacementString:(NSString *)string
{
    if (textField == _priceTf)
    {
        NSString *newString = [textField.text stringByReplacingCharactersInRange:range withString:string];

        NSString *expression = @"^([0-9]{1,6})?(\\.?\\,?([0-9]{1,2})?)?$";

        NSRegularExpression *regex = [NSRegularExpression regularExpressionWithPattern:expression
                                                                               options:NSRegularExpressionCaseInsensitive
                                                                                 error:nil];
        NSUInteger numberOfMatches = [regex numberOfMatchesInString:newString
                                                            options:0
                                                              range:NSMakeRange(0, [newString length])];
        if (numberOfMatches == 0)
            return NO;
    }

    return YES;
}