SQLite - How to use regex with if-condition?

How can I use regex with if-condition in sqlite? My query below,

....
WHERE DATE(localdate) BETWEEN '2014-10-09' AND '2015-05-12'
AND n.nid = '9'

AND CASE WHEN '9' REGEXP '^[0-9]+$' THEN w.wspdi != '-9999' ELSE w.wspdi != NULL END

error,

no such function: REGEXP

Any ideas?