Stumpy,
When you pasted Faseeh's formula above, did you by chance add an equals sign before his code? In the formula box, the text should actually read:
=AND(LEN(D6)=6,(MID(D6,3,1)="/")=TRUE,ISERROR(VALUE(LEFT(D6,2)))=FALSE,ISERROR(VALUE(RIGHT(D6,3)))=FALSE)
If you happened to paste it...