Conditional(?) regex for numbers

November 29, 2021

i was trying to create a regex that could match numbers within brackets or not, for example:

(1.000.000,00) //match
(1.000,00)     //match
(100,00)       //match
(10)           //match
(1)            //match

(2.000.000,00  //dont't match
(2.000,00      //dont't match
(200,00        //dont't match
(20            //dont't match
(2             //dont't match

3.000.000,00)  //dont't match
3.000,00)      //dont't match
300,00)        //dont't match
30)            //dont't match
3)             //dont't match

4.000.000,00   //should match
4.000,00       //should match
400,00         //should match
40             //should match
4              //should match

I need to match only numbers(in brackets or not), but only if they have all brackets (2) or none(0)

At the moment this is what i came up with: \((\d+[\.,]?)+\d*\), it matches the –match and doesn’t match the –don’t match but should match also the –should match

I’ve added the javascript tag because i’m using this regex in js and not all of the regex tokens work in the js regex constructor

I’m posting also a regex101 link

>Solution :

If supported, you can usea negative lookbehind to match either with or without parenthesis:

\(\d+(?:[.,]\d+)*\)|(?<!\S)\d+(?:[.,]\d+)*(?!\S)

\( Match (
\d+(?:[.,]\d+)* Match 1+ digits and optionally repeat matching . or , and again 1+ digits
\) Match )
| Or
(?<!\S) Negative lookbehind, assert a word boundary to the left
\d+(?:[.,]\d+)* Match 1+ digits and optionally repeat matching . or , and again 1+ digits
(?!\S) Negative lookahead, assert a whitespace boundary to the right

Regex demo

Another option could be matching optional parenthesis at both sides, and only keep the ones that have either an opening and closing parenthesis, or none.

const regex = /\(?\d+(?:[.,]?\d+)*\)?/
const strings = ["(1.000.000,00)", "(1.000,00)", "(100,00)", "(10)", "(1)", "(2.000.000,00", "(2.000,00", "(200,00", "(20", "(2", "3.000.000,00)", "3.000,00)", "300,00)", "30)", "3)", "4.000.000,00", "4.000,00", "400,00", "40", "4"];

strings.forEach(s => {

  const m = s.match(regex);
  const firstChar = s.charAt(0);
  const lastChar = s.charAt(s.length - 1);

  if (
    m &&
    (firstChar !== '(' && lastChar !== ')') ||
    firstChar === '(' && lastChar === ')'
  ) {
    console.log(s)
  }
});