SQL-Fehler: Regex error 'quantifier does not follow a repeatable item at offset 4'

Query:

        SELECT schlagwort
        FROM yyy_schlagwortliste
        JOIN yyy_dokuthekdb ON yyy_schlagwortliste.schlagwort_id = yyy_dokuthekdb.id
        WHERE schlagwort REGEXP '^(u|?)'
        GROUP BY schlagwort
        ORDER BY schlagwort