SQL & PL/SQL :: Using A Cursor Record In REGEXP_INSTR

I've got a task to star out account numbers in a free text field. The account numbers can be in a few different formats and there are a number of tables. The formats could increase or change as could the tables. For that purpose I want to use a couple of tables; one to store the formats and one to store the tablenames. The tablenames work fine, but I'm having problem with the formats.

The table definition for the formats is:-

CREATE TABLE account_format (acc_id number(2),acc_format VARCHAR2 (200), acc_replace VARCHAR2 (200),acc_process varchar2(1))
LOGGING
NOCOMPRESS
NOCACHE
NOPARALLEL
MONITORING;

and one insert would be

INSERT INTO account_format
VALUES (1,
'''[0-9]{5}[A-Z]{1}-[0-9]{5}''',
'''[0-9]{5}[A-Z]{1}-[0-9]{5}'',''****-****-****-****''',
'Y');

The idea is to bring back the format and use it in REGEXP_INSTR (or a similar REGEXP) to ascertain if the field contains the account number. The code I have been using looks a bit like this:-

CURSOR m_format_cur IS
SELECT acc.acc_format
FROM account_format acc;
m_format_rec m_format_cur%ROWTYPE;
IF REGEXP_INSTR (m_narrative, m_format_rec.acc_format) > 0 THEN
....
END IF;

I have tried several different ways to get this to work e.g. hardcoding the format and declaring a variable with the format, which both work. Whenever I try to use the value retrieved from the database field it never works. Why should a variable from a cursor not work?