再学习xpath----不推荐firepath工具取值
1.选择nodes
nodename ：通过nodename来定位，如 bookstore--选择了所有的bookstore 节点.
/ ： 从根节点开始定位，如/bookstore--选择根节点bookstore；bookstore/book--选择了bookstore下的所有book子节点. 注意：/开头代表了绝对路径。
// ： 从当前节点开始选择文档中所有匹配的节点，如//book--选择了所有的book节点，无论它们处于什么位置；bookstore//book--选择bookstore下的所有子孙book节点。
. ： 当前节点
.. ： 当前节点的父节点
@ ：选择属性，如选择所有name为lang的 属性


2. 使用条件搜索
/bookstore/book[1] : 选择bookstore子节点的第一个book 节点
注意：IE5之后起点为[0]，当时根据W3C标准，应该是从[1]开始

/bookstore/book[last()] : 选择bookstore下最后一个book子节点

/bookstore/book[last()-1] : Selects the last but one book element that is the child of the bookstore element
/bookstore/book[position()35.00] : Selects all the book elements of the bookstore element that have a price element with a value greater than 35.00
/bookstore/book[price>35.00]/title : Selects all the title elements of the book elements of the bookstore element that have a price element with a value greater than 35.00

3. 匹配搜索
* Matches any element node
@* Matches any attribute node
node() Matches any node of any kind

/bookstore/* ：Selects all the child nodes of the bookstore element
//* ：Selects all elements in the document
//title[@*] ：Selects all title elements which have any attribute

4. 选择多个
//book/title | //book/price ：Selects all the title AND price elements of all book elements
//title | //price ：Selects all the title AND price elements in the document
/bookstore/book/title | //price ：Selects all the title elements of the book element of the bookstore element AND all the price elements in the document

5. 使用XPath Axes （相对node）
ancestor ：Selects all ancestors (parent, grandparent, etc.) of the current node
ancestor-or-self ：Selects all ancestors (parent, grandparent, etc.) of the current node and the current node itself
attribute ：Selects all attributes of the current node
child ：Selects all children of the current node
descendant ：Selects all descendants (children, grandchildren, etc.) of the current node
descendant-or-self ：Selects all descendants (children, grandchildren, etc.) of the current node and the current node itself
following ：Selects everything in the document after the closing tag of the current node
following-sibling ：Selects all siblings after the current node
namespace ：Selects all namespace nodes of the current node
parent ：Selects the parent of the current node
preceding ：Selects all nodes that appear before the current node in the document, except ancestors, attribute nodes and namespace nodes
preceding-sibling ：Selects all siblings before the current node
self ：Selects the current node

定位path表达式
基本的step：
1. axis：定义所选节点和目前节点间的相对树形关系
2. node-test：识别axis中的一个节点
3. zero or more predicates：使用条件搜索
结构：axisname::nodetest[predicate]

child::book ：Selects all book nodes that are children of the current node
attribute::lang ：Selects the lang attribute of the current node
child::* ：Selects all element children of the current node
attribute::* ：Selects all attributes of the current node
child::text() Selects all text node children of the current node
child::node() Selects all children of the current node
descendant::book Selects all book descendants of the current node
ancestor::book Selects all book ancestors of the current node
ancestor-or-self::book Selects all book ancestors of the current node - and the current as well if it is a book node
child::*/child::price Selects all price grandchildren of the current node

6. XPATH运算
| :Computes two node-sets
//book | //cd --- Returns a node-set with all book and cd elements

+,-,*,div : 加减乘除

=，！= ：等和，不等

<,<=,>,>=: 小于，小于等于，大于大于等于

or， and： 与 ，或

mod: 取模

7. xpath内建功能库

给链接 暂省略：http://www.w3schools.com/xpath/xpath_functions.asp