Python re module, XPath usage

1. Summary of the usage of re regular

(1), ^ indicates which character to start with eg: ' ^g ' denotes a string starting with G. means that any character ' ^g.d ' represents a second character starting with G, and a third string of B indicates that a character appears any number of times

Import re   ' bobby123 '  '^b.*'   # Any string that begins with B appears any number of times   ^ begins with   . Any character  * Occurs any number of times if  re.match (regex_str,line    ):print('Yes  ')

(2), $ represents a string that ends with a character

' bobby123 '  '^b.*3$'   # Any string that begins with B appears any number of times   ^ with what begins   . Any character  * Occurs any number of times   $ with what character ends if  re.match (regex_strs,lines    ):print('  Yes')

The result of the operation is: ' Yes '

(3), to indicate a non-greedy pattern matching (not very clear, roughly greedy pattern matching will be from the two ends of the string to match, non-greedy from left to right, matching results will be different); () indicates that the character appears at least once

# non-greedy mode?  greedy mode does not add? Just good  'boooooooooooooobby123'. B.*?B). *'   #() Extract substring  =  re.match (regex_strs,lines)  Print(Mat.group (1))

The result of operation is: 'boooooooooooooob ';

Greedy match eg:

# + The usage character appears at least once ' boooooooooooooobbbbby123 '  '. * (b.+b). *'   #() extract substring greedy plus + result for bbb=   Re.match (regex_strs,lines) Print (Mat.group (1))

Operation result: ' BBB '

2. XPath syntax XML Instance document

We will use this XML document in the following example.

<?xml version= "1.0" encoding= "iso-8859-1"?><bookstore><book> <title  lang= "Eng" >Harry potter</title>  <price>29.99</price></book><book>  <title lang= "eng" > Learning xml</title>  <price>39.95</price></book></bookstore>

Select a node

XPath uses a path expression to select a node in the XML document. A node is selected by a path or step.

The most useful path expressions are listed below:

An expression	Description
NodeName	Select all child nodes of this node.
/	Select from the root node.
//	Selects the nodes in the document from the current node that matches the selection, regardless of their location.
.	Select the current node.
..	Selects the parent node of the current node.
@	Select the attribute.

Instance

In the table below, we have listed some path expressions and the results of the expressions:

Path Expression	Results
Bookstore	Selects all child nodes of the bookstore element.
/bookstore	Select the root element bookstore. Note: If the path starts with a forward slash (/), this path always represents the absolute path to an element!
Bookstore/book	Selects all book elements that belong to a child element of bookstore.
Book	Selects all book child elements, regardless of their position in the document.
Bookstore//book	Selects all book elements that belong to descendants of the bookstore element, regardless of where they are located under bookstore.
@lang	Select all attributes that are named Lang.

predicate (predicates)

To find a particular node or a node that contains a specified value.

The predicate is embedded in square brackets.

Instance

In the table below, we list some path expressions with predicates, as well as the results of expressions:

Path Expression	Results
/BOOKSTORE/BOOK[1]	Selects the first book element that belongs to a bookstore child element.
/bookstore/book[last ()]	Select the last book element that belongs to the bookstore child element.
/bookstore/book[last ()-1]	Select the second-to-last book element that belongs to the bookstore child element.
/bookstore/book[position () <3]	Select the first two book element that belongs to the child element of the bookstore element.
title[@lang]	Select all the title elements that have properties named Lang.
title[@lang = ' Eng ']	Selects all title elements, and these elements have the lang attribute value of Eng.
/BOOKSTORE/BOOK[PRICE>35.00]	Selects all the book elements of the bookstore element, and the value of the price element must be greater than 35.00.
/bookstore/book[price>35.00]/title	Selects all the title elements of the book element in the bookstore element, and the value of the price element must be greater than 35.00.

Select Unknown node

XPath wildcard characters can be used to select unknown XML elements.

wildcard characters	Description
*	Matches any element node.
@*	matches any attribute node.
Node ()	Matches any type of node.

Instance

In the table below, we list some path expressions and the results of these expressions:

Path Expression	Results
/bookstore/*	Selects all child elements of the bookstore element.
//*	Selects all elements in the document.
Title[@*]	Select all the title elements with attributes.

Select several paths

By using the ' | ' in a path expression operator, you can select a number of paths.

Instance

In the table below, we list some path expressions and the results of these expressions:

Path Expression	Results
Book/title | Book/price	Selects all the title and price elements of the book element.
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 that belong to the bookstore element, and all the price elements in the document.

This article source W3cschool link http://www.w3school.com.cn/xpath/xpath_syntax.asp

Python re module, XPath usage