WP_REST_Users_Controller::check_username( string $value, WP_REST_Request $request, string $param ): string|WP_Error

Source

public function check_username( $value, $request, $param ) {
	$username = (string) $value;

	if ( ! validate_username( $username ) ) {
		return new WP_Error(
			'rest_user_invalid_username',
			__( 'This username is invalid because it uses illegal characters. Please enter a valid username.' ),
			array( 'status' => 400 )
		);
	}

	/** This filter is documented in wp-includes/user.php */
	$illegal_logins = (array) apply_filters( 'illegal_user_logins', array() );

	if ( in_array( strtolower( $username ), array_map( 'strtolower', $illegal_logins ), true ) ) {
		return new WP_Error(
			'rest_user_invalid_username',
			__( 'Sorry, that username is not allowed.' ),
			array( 'status' => 400 )
		);
	}

	return $username;
}

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