r newer
    "x".replace(/x(.)?/g, (m, group) => {
      console.log(`group: ${group}`);
    });
    // group: undefined
    

Note that due to web compatibility, ` RegExp.$N ` will still return an empty
string instead of ` undefined ` ( [ bug 1053944 ](https://bugzil.la/1053944)
).

##  See also

  * [ Polyfill of many modern ` RegExp ` features ( ` dotAll ` , ` sticky ` flags, named capture groups, etc.) in ` core-js ` ](https://github.com/zloirock/core-js#ecmascript-string-and-regexp)
  * [ Regular expressions ](/en-US/docs/Web/JavaScript/Guide/Regular_expressions) guide 
  * [ Regular expressions ](/en-US/docs/Web/JavaScript/Reference/Regular_expressions)
  * [ ` String.prototype.match() ` ](/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/match)
  * [ ` String.prototype.replace() ` ](/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/replace)
  * [ ` String.prototype.split() ` ](/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/split)

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