Oracle 11g Release 2

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CREATE TABLE tests(emp_number VARCHAR(100) NOT NULL PRIMARY KEY);
INSERT INTO tests SELECT 'ASDF' FROM DUAL;
1 rows affected

INSERT INTO tests SELECT '1111' FROM DUAL;
1 rows affected

INSERT INTO tests SELECT '2222' FROM DUAL;
1 rows affected

INSERT INTO tests SELECT '3333' FROM DUAL;
1 rows affected

select * from (
    select * from tests where not regexp_like(emp_number, '[^0-9]')
) where  to_number(emp_number)  between 1 and 100
ORA-01722: invalid number

SELECT *
FROM tests
WHERE CASE WHEN NOT REGEXP_LIKE(emp_number, '\D') THEN TO_NUMBER(emp_number) END BETWEEN 1200 AND 2400


EMP_NUMBER


2222