The document that I'm expecting should only contain 'apple' or 'oranges' or 'tomatoes'. Anything but I will give the element's text a value of 'invalid', otherwise it's 'valid'.

Below expressions would still return false when 'oranges' or 'tomatoes' is present. The same answer I will get when I use "not(contains(.'apples')) or not(contains..."

.!='apple' or .!='oranges' or .!='tomatoes'

or

(.!='apple') or (.!='oranges') or (.!='tomatoes')


So I used in choose-when construct '=' instead

.='apple' or .='oranges' or .='tomatoes'

Why is != behaving differently?

The expression

.!='oranges' or .!='tomatoes'

is always true. If the value of "." is 'oranges', then .!='tomatoes' is true; if the value is 'tomatoes', then .!='oranges' is true, and if it's anything else, then both are true.

Michael Kay
http://www.saxonica.com/
Author, XSLT 2.0 and XPath 2.0 Programmer's Reference

In other words, you need

!(.='apple' or .='oranges' or .='tomatoes')

which is the equivalent of

(.!='apple' and .!='oranges' and .!='tomatoes')

Thanks for the explanation, Mike!

Old Pedant, Thanks too. I will use !(.='apple'...) construct...