Regex passes the test site but the code does not work

Question

I created a regex ( d{1,}(. d{2})?$ ) that was to allow values in the format 21 or 21.30. I tested the regex on a website and it worked:

I’m making the following code:

const regexMoneyFormat = RegExp('^\d{1,}(\.\d{2})?$');
    console.log('valor convertido pra string', price.toString())
    if (!regexMoneyFormat.test(price.toString())) {
      console.log('resultado do teste', regexMoneyFormat.test(price.toString()))
      throw new AppError('Price format invalid, valid shapes: 0 ou 0.00');
    }

Only that the values in the format I mentioned, such as the value 1 or the value 60.21 are not passing. From the print I saw that the function toString() is converting correctly, but is not going through the regex.

Why the code isn’t working?

Answer 1

The builder of RegExp takes a string, and in strings the character \ must be escaped with another \, getting \\. I mean, it should be like this:

const regexMoneyFormat = RegExp('^\\d+(\\.\\d{2})?$');

Another detail is that I switched the quantifier {1,} for + since both mean "one or more occurrences".

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Although you can also use the literal form, delimiting the expression between bars:

const regexMoneyFormat = /^\d+(\.\d{2})?$/;

So it is not necessary to escape and you can use only \.

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About regex for monetary values, it is worth taking a look here and here.

Answer 2

In accordance with the another answer already scored, you must escape the backslashes in a regular expression built from the constructor since this is a necessity of the strings themselves. And the constructor accepts strings.

Of course, you could use the literal form (as already indicated):

const regex = /^\d+(\.\d{2})?$/;

But if you eventually need to use the constructor and don’t want to escape the string (imagine a regex which has many characters that need to be escaped), one option is to use the String.raw:

const regexStr = String.raw`^\d+(\.\d{2})?$`;
const regex = new RegExp(regexStr);

Or, in a single line:

const regex = new RegExp(String.raw`^\d+(\.\d{2})?$`);

The tag String.raw, for strings template, is extremely useful in situations like this, as it does not process exhaust sequences.