src/Isbn.php
public function getGroupIdentifier() : string
    {
        if ($this->rangeInfo === null) {
            throw IsbnException::unknownGroup($this->isbn);
        }

        return $this->rangeInfo->groupIdentifier;
    }
public function getGroupIdentifier() : string
    {
        if ($this->rangeInfo === null) {
            throw IsbnException::unknownGroup($this->isbn);
        }

        return $this->rangeInfo->groupIdentifier;
    }
public function getGroupIdentifier ( ) : string { if ( $ this -> rangeInfo === null ) { throw IsbnException :: unknownGroup ( $ this -> isbn ) ; } return $ this -> rangeInfo -> groupIdentifier ; }
Returns the group identifier.

The group or country identifier identifies a national or geographic grouping of publishers.

For ISBN-13, the identifier includes the GS1 (EAN) prefix, for example "978-2".
For the equivalent ISBN-10, the identifier would be "2".

@return string

@throws IsbnException If this ISBN is not in a recognized group.
Returns the group identifier .