RegExp[Symbol.species]

The value of the constructor (this) on which get [Symbol.species] was called. The return value is used to construct copied RegExp instances.

The [Symbol.species] accessor property returns the default constructor for RegExp objects. Subclass constructors may override it to change the constructor assignment. The default implementation is basically:

// Hypothetical underlying implementation for illustration
class RegExp {
  static get [Symbol.species]() {
    return this;
  }
}

Because of this polymorphic implementation, [Symbol.species] of derived subclasses would also return the constructor itself by default.

class SubRegExp extends SubRegExp {}
SubRegExp[Symbol.species] === SubRegExp; // true

Some RegExp methods create a copy of the current regex instance before running exec(), so that side effects such as changes to lastIndex are not retained. The [Symbol.species] property is used to determine the constructor of the new instance. The methods that copy the current regex instance are:

• [Symbol.matchAll]()
• [Symbol.split]()

The [Symbol.species] property returns the default constructor function, which is the RegExp constructor for RegExp objects:

RegExp[Symbol.species]; // function RegExp()

In an instance of a custom RegExp subclass, such as MyRegExp, the MyRegExp species is the MyRegExp constructor. However, you might want to overwrite this, in order to return parent RegExp objects in your derived class methods:

class MyRegExp extends RegExp {
  // Overwrite MyRegExp species to the parent RegExp constructor
  static get [Symbol.species]() {
    return RegExp;
  }
}

Or you can use this to observe the copying process:

class MyRegExp extends RegExp {
  constructor(...args) {
    console.log("Creating a new MyRegExp instance with args:", args);
    super(...args);
  }
  static get [Symbol.species]() {
    console.log("Copying MyRegExp");
    return this;
  }
  exec(value) {
    console.log("Executing with lastIndex:", this.lastIndex);
    return super.exec(value);
  }
}

Array.from("aabbccdd".matchAll(new MyRegExp("[ac]", "g")));
// Creating a new MyRegExp instance with args: [ '[ac]', 'g' ]
// Copying MyRegExp
// Creating a new MyRegExp instance with args: [ MyRegExp /[ac]/g, 'g' ]
// Executing with lastIndex: 0
// Executing with lastIndex: 1
// Executing with lastIndex: 2
// Executing with lastIndex: 5
// Executing with lastIndex: 6

• RegExp
• Symbol.species