Quick repair ...
Can be repaired using crazy glue or windshield filler as the filler. To prepare ruler by wrapping two popcicle sticks with the the cling strips we use for marking our rules(will make glue not stixk to it. Then tape to form the corners back. Place another piece of the cling strip under ruler on flat surface and fill the corner with a tube of crazy glue or get out of automotive the windshield repair kit this will make it a corner again. It will take about 2 days for it to get hard then remove sticks and file smooth.
I have done this one and still use the ruler to day. But before fixing the broken area always seem to snag my fabric.
Hope this helps you.