BANNER
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Oracle Database 12c Standard Edition Release 12.1.0.2.0 - 64bit Production

SQL> CREATE USER A3 IDENTIFIED BY p3;
CREATE USER A3 IDENTIFIED BY p3
            *
ERROR at line 1:
ORA-65096: invalid common user or role name


SQL> CREATE USER testUser IDENTIFIED BY pass;
CREATE USER testUser IDENTIFIED BY pass
            *
ERROR at line 1:
ORA-65096: invalid common user or role name


SQL> CREATE USER c##testUser IDENTIFIED BY pass;

User created.

SQL>