The first thing to note is that your split operation doesn't split words, as you don't use any inter-word gap character in your split operation.
Try adding space to the Split array and see if that improves things.
Then use the debugger to see exactly what you are checking.
Personally, I wouldn't use a regex for this - I'd use String.ToLower on the input, and then use
Enumerable.Contains[
^] to see if this word is in the list.