Que veut dire l'acronyme ”??” ?

Fatal error: Uncaught exception 'PDOException' with message 'SQLSTATE[42000]: Syntax error or access violation: 1139 Got error 'repetition-operator operand invalid' from regexp' in /home/meltingm/www/meltingmots.com/functions/get_relative_items.php:28 Stack trace: #0 /home/meltingm/www/meltingmots.com/functions/get_relative_items.php(28): PDO->query('SELECT * FROM a...') #1 /home/meltingm/www/meltingmots.com/fiche_abr.php(369): get_relative_items('??', '2', 'abreviation', 'mot_abr') #2 {main} thrown in /home/meltingm/www/meltingmots.com/functions/get_relative_items.php on line 28