Escape square brackets when assigning a class name to an element
The introduction of jQuery API page gives the above explanation about this:
To use any of the meta-characters ( such as !"#$%&'()*+,./:;<=>?@[]^`{|}~ ) as a literal part of a name, it must be escaped with with two backslashes: \\.
Escape with TWO (2) backslashes.
http://learn.jquery.com/using-jquery-core/faq/how-do-i-select-an-element-by-an-id-that-has-characters-used-in-css-notation/