# Assume sufficiently advanced Regex is copied

I came across a snippet of javascript code today to add commas to a number.


```javascript
let commaNum = numStr.replace(/\B(?=(\d{3})+(?!\d))/g, ",");
``` 

As soon as I saw it, I suspected someone had copied it from somewhere else. After some digging I found I was right and it had been copied multiple times for years. I can find it in  [StackOverflow](https://stackoverflow.com/questions/2254185/regular-expression-for-formatting-numbers-in-javascript)  and  [blogs](https://www.delftstack.com/howto/javascript/javascript-add-commas-to-number/) which leads me to my Law of Regex...


> Assume sufficiently advanced Regex is copied


which is fine but please also link to where the regex was copied. Eventually someone will track down a bug to the regex like I did with this regex.
If you have more than three decimal places...


```javascript

"1234.1234".replace(/\B(?=(\d{3})+(?!\d))/g, ",");
//'1,234.1,234'
``` 
The regex(and bug) had been copied around and that bug had been commented on but if there was a reference in a comment, it could have saved me little extra time and potentially more Devs in the future also.

PS. Modern browsers handle this a better way with internationalization now