【Advanced Section】Advanced Data Parsing: XPath and Regular Expressions - Advanced: Extracting Complex Data with Regular Expressions

# Python Web Crawler Development Collection

## 1. XPath Syntax and Selectors

### 1.1 Basic XPath Syntax

XPath (XML Path Language) is a language used for navigating and selecting nodes in XML documents. Its basic syntax follows the format:

/root-element/child-element/sub-element/...

Where:

* `/`: Root node
* `root-element`: Document root element
* `child-element`: Child element of the root element
* `sub-element`: Sub-element of the child element

### 1.2 XPath Selector Types

XPath provides various selector types to select specific nodes in an XML document:

***Element Selector**: Selects elements with a specific name. For example, `/book` selects the element named "book".
***Attribute Selector**: Selects elements with a specific attribute. For example, `/book[@id="1"]` selects the "book" element with an attribute `id` value of "1".
***Child Element Selector**: Selects elements with a specific child element. For example, `/book/author` selects the "book" element with a "author" child element.
***Descendant Selector**: Selects all descendant elements of an element. For example, `/book//author` selects all "author" descendant elements of "book" elements.

## 2. Advanced XPath Applications

### 2.1 XPath Syntax and Selectors

#### 2.1.1 Basic XPath Syntax

XPath (XML Path Language) is a language used for navigating and selecting nodes in XML documents. Its basic syntax is as follows:

/root-element/child-element/grandchild-element/...

Where:

* `/` signifies an absolute path starting from the root element.
* `root-element` is the root element of the XML document.
* `child-element` is a child element of the root element.
* `grandchild-element` is a child element of the child element, and so on.

#### 2.1.2 XPath Selector Types

XPath provides various selector types for selecting specific elements:

***Node Selector**: Selects nodes of a specific type, such as elements, attributes, or text nodes.
***Attribute Selector**: Selects elements with specific attributes.
***Predicate Selector**: Selects elements that satisfy specific conditions.
***Axis Selector**: Selects elements associated with the current node, such as parent, child, or sibling nodes.

### 2.2 XPath Functions and Operators

#### 2.2.1 XPath Function Categories

XPath offers a rich set of functions for operating on and transforming XML data, mainly categorized as follows:

***String Functions**: Process string data, e.g., `substring()`, `concat()`.
***Number Functions**: Process numeric data, e.g., `sum()`, `average()`.
***Boolean Functions**: Return boolean values, e.g., `true()`, `false()`.
***Node Functions**: Operate on XML nodes, e.g., `count()`, `position()`.

#### 2.2.2 XPath Operators

XPath also provides operators for comparing, combining, and modifying data:

***Comparison Operators**: Compare two values, e.g., `=`, `!=`, `<`, `>`.
***Logical Operators**: Combine boolean values, e.g., `and`, `or`, `not`.
***Arithmetic Operators**: Perform arithmetic operations, e.g., `+`, `-`, `*`, `/`.

### 2.3 XPath Practical Examples

#### 2.3.1 Extracting Specific Data from an XML Document

**Code Block:**

<root>
  <item>
    <name>Item 1</name>
     <price>10</price>
   </item>
   <item>
     <name>Item 2</name>
     <price>20</price>
   </item>
</root>

/root/item[name='Item 2']/price

**Lo