Validating Currency on Javascript using a regex not working

Question

I need a regular expression for currency type. Requirements :

1. First char can be a '$'. It can appear either 0 or 1 times.
2. Then a digit can appear multiple times.
3. Then a decimal point can appear either 0 or 1 time.
4. After this, a digit can appear 0 or more times. 

I have written the following regular expression :

\$?\d+\.?\d*

I need to test this on JS . This is how i test this on JS;

var str = "$cng";
var patt = new RegExp('[\$?\d+\.?\d*]');
var res = patt.test(str);
console.log(res);

The above string $cng is returning true. I am not able to get it. Am i missing anything here. Can anyone please help. Thanks in advance.

Answer 1

1. You must need to escape all the backslashes one more times when passing it to the RegExp constructor which has double quotes as delimiter.

2. And also i suggest you to remove the square brackets around your pattern.

So change your pattern like below,

var patt = new RegExp("^\\$?\\d+\\.?\\d*$");

OR

var patt = new RegExp("^\\$?\\d+(?:\\.\\d+)?$");

Example:

> var str = "$123.12";
undefined
> var patt = new RegExp("^\\$?\\d+(?:\\.\\d+)?$");
undefined
> patt.test(str);
true
> var patt = new RegExp("^\\$?\\d+(?:\\.\\d+)?$");
undefined
> patt.test('$123.12$');
false

Answer 2

[\$?\d+\.?\d*]==>[] is a character class.

Your regex will just match 1 character out of the all defined inside the character class.

Use

^\\$?\\d+\\.?\\d*$

or

/^\$?\d+\.?\d*$/ 

to be very safe.

Answer 3

Replace RegExp('[\\$?\\d+\\.?\\d*]') with RegExp(/\\$?\\d+\\.?\\d*/) and it will work as expected.

Working Code Snippet:

 var str = "$100.10"; var patt = new RegExp(/^\\$?\\d+\\.?\\d*$/); var res = patt.test(str); console.log(res); 

EDIT:

You can also simply do: var res = /^\\$?\\d+\\.?\\d*$/.test(str);

Answer 4

Your regular expression should also match the beginning and end of the string, otherwise it will only test if the string contains a currency:

^\$?\d+\.?\d*$

You added brackets around the regular expression when you implemented it in Javascript, which changes the meaning entirely. The pattern will find a match if any of the characters within the brackets exist in the string, and as there is a $ in the string the result was true .

Also, when you have a regular expression in a string, you have to escape the backslashes:

var patt = new RegExp('^\\$?\\d+\\.?\\d*$');

You can also use a regular expression literal to create the object:

var patt = /^\$?\d+\.?\d*$/;

---

You might want to change the requirements so that the decimal point only is allowed if there are digits after it, so that for example $1. is not a valid value:

^\$?\d+(\.\d+)?$