Borrowing from CSS 1–3, and then adding its own, jQuery offers a powerful set of tools for matching a set of elements in a document.
To use any of the meta-characters ( such as!"#$%&'()*+,./:;<=>?@[\]^`{|}~) as a literal part of a name, it must be escaped with with two backslashes:\\. For example, an element withid="foo.bar", can use the selector$("#foo\\.bar"). The W3C CSS specification contains thecomplete set of rules regarding valid CSS selectors. Also useful is the blog entry by Mathias Bynens onCSS character escape sequences for identifiers.
Selects all the elements that are the nth-child of their parent in relation to siblings with the same element name, counting from the last element to the first.