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C# Scraping HTML Links
Scraping HTML extracts important page elements. It has many uses for webmasters and ASP.NET developers. With the Regex type and WebClient, we implement screen scraping for HTML.RegexWebClient
First, we will scrape HTML links from Wikipedia.org. This is permitted by Wikipedia's GPL license, and this demonstration is fair use. Here we see code that downloads the English Wikipedia page.
Note: It opens Wikipedia and downloads the content at the specified URL. Part 2 uses my special code to loop over each link and its text.
C# program that scrapes HTML
using System.Diagnostics;
using System.Net;
class Program
{
static void Main()
{
// 1.
// URL: http://en.wikipedia.org/wiki/Main_Page
WebClient w = new WebClient();
string s = w.DownloadString("http://en.wikipedia.org/wiki/Main_Page");
// 2.
foreach (LinkItem i in LinkFinder.Find(s))
{
Debug.WriteLine(i);
}
}
}
Example 2. Here I show a simple class that receives the HTML string and then extracts all the links and their text into structs. It is fairly fast, but I offer some optimization tips further down. It would be better to use a class.Class
MatchCollection: This example first finds all hyperlink tags. We store all the complete A tags into a MatchCollection.
Step 2: The code loops over all hyperlink tag strings. In the algorithm, the next part examines all the text of the A tags.
HREF: This attribute points to other web resources. This part is not failsafe, but almost always works.
Returns: The method returns the List of LinkItem objects. This list can then be used in the foreach-loop from the first C# example.
C# program that scrapes with Regex
using System.Collections.Generic;
using System.Text.RegularExpressions;
public struct LinkItem
{
public string Href;
public string Text;
public override string ToString()
{
return Href + "\n\t" + Text;
}
}
static class LinkFinder
{
public static List<LinkItem> Find(string file)
{
List<LinkItem> list = new List<LinkItem>();
// 1.
// Find all matches in file.
MatchCollection m1 = Regex.Matches(file, @"(<a.*?>.*?</a>)",
RegexOptions.Singleline);
// 2.
// Loop over each match.
foreach (Match m in m1)
{
string value = m.Groups[1].Value;
LinkItem i = new LinkItem();
// 3.
// Get href attribute.
Match m2 = Regex.Match(value, @"href=\""(.*?)\""",
RegexOptions.Singleline);
if (m2.Success)
{
i.Href = m2.Groups[1].Value;
}
// 4.
// Remove inner tags from text.
string t = Regex.Replace(value, @"\s*<.*?>\s*", "",
RegexOptions.Singleline);
i.Text = t;
list.Add(i);
}
return list;
}
}
Tests. My first two attempts at this code were incorrect and had unacceptable bugs, but the version shown here works. You need to use RegexOptions.SingleLine. The dot in a Regex matches all characters except a newline unless this is specified.
Tip: To match multiline links, we require RegexOptions.Singleline. This is an important option.
RegexOptions.MultilineTest the program on your website. It prints out matches to the console. Here we see part of the current results for the Wikipedia home page. The original HTML shows where the links were extracted. They are contained in a LI tag.
Note: You will see my program successfully extracted the anchor text and also the HREF value.
Output
#column-one
navigation
#searchInput
search
/wiki/Wikipedia
Wikipedia
/wiki/Free_content
free
/wiki/Encyclopedia
encyclopedia
/wiki/Wikipedia:Introduction
anyone can edit
/wiki/Special:Statistics
2,617,101
/wiki/English_language
English
/wiki/Portal:Arts
Arts
/wiki/Portal:Biography
Biography
/wiki/Portal:Geography
Geography
/wiki/Portal:History
History
/wiki/Portal:Mathematics
Mathematics
/wiki/Portal:Science
Science
/wiki/Portal:Society
Society
/wiki/Portal:Technology_and_applied_sciences
Technology
Original website HTML
<ul>
<li><a href="/wiki/Portal:Arts" title="Portal:Arts">Arts</a></li>
<li><a href="/wiki/Portal:Biography" title="Portal:Biography">Biography</a></li>
<li><a href="/wiki/Portal:Geography" title="Portal:Geography">Geography</a></li>
</ul>
SingleLine. SingleLine is an important option. Microsoft states that SingleLine "Specifies single-line mode. Changes the meaning of the dot so it matches every character (instead of every character except \n)."
Performance. You can improve performance of the regular expressions by specifying RegexOptions.Compiled, and also use instance Regex objects, not the static methods I show. Normally, your Internet connection will be the bottleneck.
Summary. We scraped HTML content from the Internet. The code is more flexible than some other approaches. Using three regular expressions, you can extract HTML links into objects with a fair degree of accuracy.
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- C# Parallel.For Example (Benchmark)
- C# Parallel.Invoke: Run Methods on Separate Threads
- C# Parameter Optimization
- C# Parameter Passing, ref and out
- C# params Keyword
- C# int.Parse: Convert Strings to Integers
- C# partial Keyword
- C# Path.ChangeExtension
- C# Path Exists Example
- C# Path.GetExtension: File Extension
- C# Path.GetRandomFileName Method
- C# Pragma Directive
- C# Predicate (Lambda That Returns True or False)
- C# Pretty Date Format (Hours or Minutes Ago)
- C# PreviewKeyDown Event
- C# Random Lowercase Letter
- C# Random Paragraphs and Sentences
- C# Random String
- C# Random Number Examples
- C# StreamReader ReadLine, ReadLineAsync Examples
- C# readonly Keyword
- C# Recursion Example
- C# Regex, Read and Match File Lines
- C# Regex Groups, Named Group Example
- C# Regex.Matches Quote Example
- C# Regex.Matches Method: foreach Match, Capture
- C# Regex.Replace, Matching End of String
- C# Regex.Replace, Remove Numbers From String
- C# Regex.Replace, Merge Multiple Spaces
- C# Regex.Replace Examples: MatchEvaluator
- C# Regex.Split, Get Numbers From String
- C# Regex.Split Examples
- C# Regex Trim, Remove Start and End Spaces
- C# RegexOptions.Compiled
- C# RegexOptions.IgnoreCase Example
- C# RegexOptions.Multiline
- C# Region and endregion
- C# Remove Char From String at Index
- C# Remove Element
- C# Remove HTML Tags
- C# Remove String
- C# Reserved Filenames
- C# return Keyword
- C# Reverse String
- C# Reverse Words
- C# Reverse Extension Method
- C# RichTextBox Example
- C# Right String Part
- C# RNGCryptoServiceProvider Example
- C# ROT13 Method, Char Lookup Table
- C# SelectMany Example: LINQ
- C# Sentinel Optimization
- C# SequenceEqual Method (If Two Arrays Are Equal)
- C# Shift Operators (Bitwise)
- C# Short and ushort Types
- C# Single Method: Get Element If Only One Matches
- C# SingleOrDefault
- C# Singleton Pattern Versus Static Class
- C# Singleton Class
- C# sizeof Keyword
- C# Skip and SkipWhile Examples
- C# Sleep Method (Pause)
- C# Sort Dictionary: Keys and Values
- C# Sort by File Size
- C# Sort, Ignore Leading Chars
- C# Sort KeyValuePair List: CompareTo
- C# Sort Strings by Length
- C# Thread.SpinWait Example
- C# Math.Sqrt Method
- C# stackalloc Operator
- C# StackOverflowException
- C# StartsWith and EndsWith String Methods
- C# Static Array
- C# Static Dictionary
- C# Stopwatch Examples
- C# Stream
- C# StreamReader ReadToEnd Example (Read Entire File)
- C# StreamReader ReadToEndAsync Example (Performance)
- C# StreamReader Examples
- C# StreamWriter Examples
- C# String Append (Add Strings)
- C# String Compare and CompareTo Methods
- C# String Constructor (new string)
- C# string.Copy Method
- C# CopyTo String Method: Put Chars in Array
- C# Empty String Examples
- C# String Equals Examples
- C# String For Loop, Count Spaces in String
- C# string.Intern and IsInterned
- C# String IsUpper, IsLower
- C# String Length Property: Get Character Count
- C# String Literal: Newline and Quote Examples
- C# StringBuilder Capacity
- C# StringBuilder Clear (Set Length to Zero)
- C# StringBuilder Data Types
- C# StringBuilder Performance
- C# StringBuilder Equals (If Chars Are Equal)
- C# StringBuilder Memory
- C# StringBuilder ToString: Get String From StringBuilder
- C# StringWriter Class
- C# Sum Method: Add up All Numbers
- C# Switch Char, Test Chars With Cases
- C# Switch Enum
- C# System (using System namespace)
- C# Tag Property: Windows Forms
- C# TextInfo Examples
- C# TextReader, Returned by File.OpenText
- C# TextWriter, Returned by File.CreateText
- C# this Keyword
- C# ThreadPool
- C# Thread Join Method (Join Array of Threads)
- C# ThreadPool.SetMinThreads Method
- C# TimeZone Examples
- C# Get Title From HTML With Regex
- C# ToArray Extension Method
- C# ToCharArray: Convert String to Array
- C# ToDictionary Method
- C# Token
- C# ToList Extension Method
- C# ToLookup Method (Get ILookup)
- C# ToLower and ToUpper: Uppercase and Lowercase Strings
- ToolStripContainer Control: Dock, Properties
- C# ToolTip: Windows Forms
- C# ToString Integer Optimization
- C# ToString: Get String From Object
- C# ToTitleCase Method
- C# TrackBar: Windows Forms
- C# Tree and Nodes Example: Directed Acyclic Word Graph
- C# TreeView Tutorial
- C# Trim Strings
- C# Thread Methods
- C# History
- C# Features
- C# Variables
- C# Data Types
- C# Operators
- C# Keywords
- C# New Features | C# Version Features
- C# Programs
- C# Program to swap numbers without third variable
- C# Program to convert Decimal to Binary
- C# Program to Convert Number in Characters
- C# Program to Print Alphabet Triangle
- C# Program to print Number Triangle
- C# Program to generate Fibonacci Triangle
- C# String Compare() method
- C# String CompareOrdinal() method
- C# String CompareTo() method
- C# String Concat() method
- C# String Contains() method
- C# String CopyTo() method
- C# String EndsWith() method
- C# String Equals() method
- C# String Format() method
- C# String IndexOf() method
- C# String Insert() method
- C# String Intern(String str) method
- C# String IsInterned() method
- C# String Normalize() method
- C# String IsNullOrEmpty() method
- C# String IsNullOrWhiteSpace() method
- C# String Join() method
- C# String LastIndexOf() method
- C# String LastIndexOfAny() method
- C# String PadLeft() method
- C# String PadRight() method
- C# Nullable
- C# String Remove() method
- C# String Replace() method
- C# String Split() method
- C# String StartsWith() method
- C# String SubString() method
- C# Partial Types
- C# Iterators
- C# Delegate Covariance
- C# Delegate Inference
- C# Anonymous Types
- C# Extension Methods
- C# Query Expression
- C# Partial Method
- C# Implicitly Typed Local Variable
- C# Object and Collection Initializer
- C# Auto Implemented Properties
- C# Dynamic Binding
- C# Named and Optional Arguments
- C# Asynchronous Methods
- C# Caller Info Attributes
- C# Using Static Directive
- C# Exception Filters
- C# Await in Catch Finally Blocks
- C# Default Values for Getter Only Properties
- C# Expression Bodied Members
- C# Null Propagator
- C# String Interpolation
- C# nameof operator
- C# Dictionary Initializer
- C# Pattern Matching
- C# Tuples
- C# Deconstruction
- C# Local Functions
- C# Binary Literals
- C# Ref Returns and Locals
- C# Expression Bodied Constructors and Finalizers
- C# Expression Bodied Getters and Setters
- C# Async Main
- C# Default Expression