It's a rather complicated way to ensure that all points have y coordinate between 2 and 4, thus cutting all lines that would extend out of the "writable" two squares.

That expression equals zero if and only if 2<=y<=4.

(edit) If you try "L", the formula contains a product member (x-2) that would produce a vertical line whose points all have x=2, containing the vertical segment of "L". The final string excludes that.

The empty string does not.

http://www.xamuel.com/inverse-graphing-calculator.php?phrase...

But, I have no idea. Perhaps it is a bug wherein a CRLF is being added to the internal string? However, I highly doubt that.