Usage: Just enter ISBN in box and validation image will pop up.
Be sure whether you want to check 10 digits or 13 digits.
False negative or false positive
Don't take this seriously it may fail at some point. I'll do my best to fix it.
I recommend you don't use it for prefix 979 just for 978
There's is ISBN.org/isbn_converter
or some other site that does what you need.