== Why is this an issue?

When using regular expressions, a capturing group provides extra information about the matched pattern. Named groups will store the matched contents on the `groups` property of the returned matches.

[source,javascript]
----
const regex = /(?<month>[0-9]{2})\/(?<year>[0-9]{2})/;
const { groups: {month, year} } = regex.exec("01/02"); // month is '01', year is '02'
----

This rule raises issues in several cases:

* Named groups are defined but never called by their name. To make the code more readable, remove unused named capturing groups.

[source,javascript]
----
const score = "14:1";
const scorePattern = /(?<player1>[0-9]+):(?<player2>[0-9]+)/; // Noncompliant - named groups are never used

if (scorePattern.exec(score)) {
  checkScore(score);
}
----

* Named groups are defined but called by their index. To make the code more readable:
** access the matched contents using the named group; or
** remove it from the pattern.

[source,javascript]
----
const datePattern = /(?<month>[0-9]{2})/(?<year>[0-9]{2})/; 
const dateMatcher = datePattern.exec("01/02");

if (dateMatcher) {
  checkValidity(dateMatcher[1], dateMatcher[2]);  // Noncompliant - group indexes are used instead of names
}
----

* Named groups are referenced while not defined. This can have undesired effects, as it will return `undefined` if there are other named groups. If there are none, `groups` will be `undefined`, and trying to access the named group will throw a `TypeError` if its existence has not been checked.

[source,javascript]
----
const datePattern = /(?<month>[0-9]{2})/(?<year>[0-9]{2})/; 
const dateMatcher = datePattern.exec("01/02");

if (dateMatcher) {
  checkValidity(dateMatcher.groups.day); // Noncompliant - there is no group called "day", returns `undefined`
}

----



== Resources

=== Documentation

* https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Regular_expressions[MDN - Regular expressions]
* https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Regular_expressions/Groups_and_backreferences[MDN - Groups and backreferences]