Text Check - Color And Word Order

I recently started wondering about how Microsoft would go about checking the text and checking its color, word order (spell check), etc. and I was trying to think of how they would do it if they were forced to write something, like Word or the text editor in Visual Studio, in Visual Basic.NET. Ways that I thought of could be using an array, setting a string to a word in that array, and check if that word is in the text. However, after I tried it, I realized I had to set the .SelectionStart property to zero, which got annoying if the document was long. Also, with arrays, though I could store the strings in a text file and read from that, I was limited to the number of items I could have, and I'm sure it would get frustrating if I added or subtracted a word (then having to back in the code and replace all of the instances of that array.)

Then I started thinking about streams. Though I have not tried them yet, I would imagine that if they had to be used, the problem of speed would come up again. But what if the stream only checked say 30 characters before and after the current selection start? Well, that would solve the speed problem, but it would bring up another. Every time the caret position changed, (if we were checking for color changing) we would be limited to those 60 characters. Say the word is "hello". What if "hello" began 31 characters behind the current SelectionStart? "h" would remain the same color, but "ello" would turn the default color (this is because we would have to reset the color everytime just in case the user changed a certain word.)