The X-definition below describes the root XML element "Inventory", which contains the "Book" elements. Each "Book" element must have an "ISBN" attribute corresponding to the pattern and may optionally contain a year of publication. A "Book" element may contain multiple "Author" elements and must contain a "Title" element. Information on ISBNs, number of books and errors is printed on the system output as follows.
The input data shows the use of this definition for two books, one with multiple authors and the other with only the title.
Note XML allows to use of both " and ' characters as a delimiter of attribute value you can reverse usage of them (you can try it here).