regular expression used to insert a certain character (like a comma) at every group of three digits in a number

regNumber.js

// reg: /\B(?=(\d{3})+(?!\d))/g

let number = "1234567890";
let formattedNumber = number.replace(/\B(?=(\d{3})+(?!\d))/g, " ");
console.log(formattedNumber);

// return 1 234 567 890

    // "/": Delimiters used to indicate the beginning and end of the regular expression.

    // "\B": This is a word boundary assertion, which matches a position that is not a word boundary. In this context, it's used to ensure that the following pattern only matches within a number and not at the beginning or end.

    // "(?=(\d{3})+(?!\d))": This is a positive lookahead assertion, which is a way to check if a certain pattern follows the current position without including it in the match.

    //     "(\d{3})+": This part matches groups of three digits. \d represents a digit, and {3} specifies that exactly three digits should be matched. The + outside the parentheses means that this group can be repeated one or more times.

    //     "(?!\d)": This is a negative lookahead assertion, ensuring that the three-digit group is not followed by another digit. It's a way to make sure that the match occurs at the right place in a number, without including the fourth digit.

    // "/g:" This flag stands for global, and it means that the regular expression should be applied globally to the entire string, rather than stopping after the first match.