Application Express :: Invalid Number Error

REVNUMBER is a VARCHAR2. I've set it to increment by letter in alphabet to note the revision of a document, i.e. IR*, A, B, C... . This works beautifully.

VERNUMBER is a VARCHAR2. I've set it to increment by number but starts from null to 'IR' to 1, 2, 3...

Until now, I've been incrementing VERNUMBER without problem if, instead of IR as the first incremented value I leave the field null. The user has asked that IR display. I've tried using IR as the default but I still need to increment from there so I don't see it making a difference between that or dumping it as the first condition of the following. The problem is I get an invalid number error either way.

*'IR' = 'Initial Release'

SELECT CASE
WHEN VERNUMBER IS NULL THEN 'IR'
WHEN VERNUMBER = 'IR' THEN '1'
ELSE REGEXP_REPLACE(VERNUMBER, '[0-9]+$', REGEXP_SUBSTR(UPPER(VERNUMBER), '[0-9]+$') + 1)
END NEW_VERSION
FROM   DOC_INFO
WHERE  DOC_INFO_ID = :P3_DOC_INFO_ID