If the bookstore JSON is modified to include a book with a square bracket in the title (which is the key in the model), that book cannot be queried. Here is such sample book:

"books" : [
  {
    "title": "Catching Fire: The Hunger Games [Book 2]",
    "lang": "English",
    "authors": ["Suzanne Collins"],
    "pub_year": 2009,
    "price": 29
  }
] 

Investigation shows that this data is being written to the database with xpath: 

/bookstore/categories[@code='5']/books[@title='Catching Fire: The Hunger Games [Book 2]']

A query such as '//books' will not include this result, nor other queries such as '//books[@title="Catching Fire: The Hunger Games [Book 2]"]'

Note the problem seems to also occur if a non-key attribute contains square brackets:

'//books/authors[text()="Joe Bloggs [translator]"]'

This needs further testing to confirm.

Proposed Solution

When data nodes are being parsed, square brackets could be escaped e.g. /bookstore

/categories[@code='5']/books[@title='Catching Fire: The Hunger Games \[Book 2\]']

The query regex would require modification to support this.