SQL> CREATE TABLE tab1 (
  2    name VARCHAR2(30)
  3      CONSTRAINT name_not_alfa CHECK (TRIM(TRANSLATE(name,
  4                 ' 0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ',
  5                 '                                                               '))
  6                 IS NULL)
  7  );
Table created.

SQL>
SQL> INSERT INTO tab1 VALUES ('Alfanumeric value 1 2 3');
1 row created.

SQL> INSERT INTO tab1 VALUES ('Non-Alfanumeric value!');
INSERT INTO tab1 VALUES ('Non-Alfanumeric value!')
*
ERROR at line 1:
ORA-02290: check constraint (HR.NAME_NOT_ALFA) violated