SQL> CREATE TABLE TBL(REGION_CODE CHAR(12 CHAR),CONSTRAINT REGION_CODE_CK CHECK (LENGTH(REGION_CODE)=12));

Table created.

SQL> insert into tbl values('123');

1 row created.

SQL> insert into tbl values(null);

1 row created.

SQL> drop table tbl purge;

Table dropped.

SQL> CREATE TABLE TBL(REGION_CODE VARCHAR2(12 CHAR) NOT NULL,
  2  CONSTRAINT REGION_CODE_CK CHECK (LENGTH(REGION_CODE)=12));

Table created.

SQL> insert into tbl values('123');
insert into tbl values('123')
*
ERROR at line 1:
ORA-02290: check constraint (SCOTT.REGION_CODE_CK) violated


SQL> insert into tbl values('123456789012');

1 row created.

SQL>

CREATE TABLE TBL(REGION_CODE CHAR(12 CHAR),
CONSTRAINT REGION_CODE_CK CHECK (LENGTH(NVL(REPLACE(REGION_CODE,' '),'X')) = 12 OR REGION_CODE IS NULL));