Assuming that your string date is in 'yyyymmdd' format then you could do:

SELECTTestDate,

ISDATE(TestDate) AS [IsDate?],

CASE WHEN TestDate LIKE '[12][0-9][0-9][0-9][01][0-9][0-3][0-9]'

THEN 'RegExOK' ELSE 'RegExBad' END,

CASE WHEN TestDate LIKE '[12][0-9][0-9][0-9][01][0-9][0-3][0-9]'

AND IsDate(TestDate) = 1

THEN 'Valid' ELSE 'Invalid' END

FROM

(

SELECT'2000' AS [TestDate]-- Valid year of IsDate() but fails RegEx

UNION ALL SELECT '20001231'-- Valid

UNION ALL SELECT '20000231'-- Passes reges, Feb. does not have 31 days!

UNION ALL SELECT '20009999'-- Fails regex

) AS X

TestDate IsDate?

-------- ----------- -------- -------

2000 1 RegExBad Invalid

20001231 1 RegExOK Valid

20000231 0 RegExOK Invalid

20009999 0 RegExBad Invalid