Question

I have the following method:

protected BigDecimal stringToBigDecimal(String value) throws ParseException
{
    if (Pattern.matches("[\\d,.]+", value))
    {
        Number n = NumberFormat.getInstance().parse(value);

        return new BigDecimal(n.doubleValue());
    }
    else
    {
        throw new ParseException("Invalid String", -1);
    }
}

This value throws an exception (as expected), so this regExpr works:

String wrongAmount = "1,U35,345.43";

But when I try this:

protected BigDecimal stringToBigDecimal(String value) throws ParseException
{
    if (Pattern.matches("[\\D&&[^.,]]+", value))
    {
        throw new ParseException("Invalid String", -1);
    }

    Number n = NumberFormat.getInstance().parse(value);

    return new BigDecimal(n.doubleValue());
}

No exception is thrown, but why?
First I thought that there is a logical problem with the “except for” expression so I tried this value with this pattern:

String wrongAmount = "1U3534543";

protected BigDecimal stringToBigDecimal(String value) throws ParseException
{
    if (Pattern.matches("[\\D]+", value))
    {
        throw new ParseException("Invalid String", -1);
    }

    Number n = NumberFormat.getInstance().parse(value);

    return new BigDecimal(n.doubleValue());
}

…but this doesn’t match? What did I understand wrong on the “non-digit expression” \\D.. The value contains an “U”, so why doesn’t it match?

Answer 1

You are not properly negating you expressions.

In the first example, you say “if my input matches only digits, commas and dots”, then it’s ok.

In your second example (and I am not sure that it does what you meant but that’s another story), you meant to say “if the input matches only non-digits, non-commas, non-dots chars”, but your input contains digits, so the match won’t work.

In your last example, same thing, you are saying “if the input matches only non-digit chars” but again you have digits in your input