I skimmed through it. Plenty of clearly written examples and good
easy-to-read explanations. It could use a bit of grammatical checking
though.
The example with "There is a animal in your house" would be more
demonstrative of which of the 4 lines evaluated falsely if there was
"line 1-4" or part of the match data in the output.
Your mathematical proof contains a zero division error.
is.gd URL shortener has banned I Love Ruby. Don't know why exactly. One
of it terms states that is.gd links should not be posted in forums. So
possibly that might have caused the problem (as I have posted it here in
Ruby forum).