I like the code from choroba, he posted that code while I was still writing mine.

Some notation comments: Instead of "ChkABN", I would use "isABN". That is a common convention that makes it clear that this is a yes/no question, usually >0 or 0 for return value (any non zero value is "true").

I would reserve any variable name that starts with a capital letter for a Class.

I personally like the style that choroba use for naming (all lower case and _ for separation). is_ABN() would be a good name for me.