Regular expressions in C#

Before I started my Web Crawler project I used always wonder how you’d go about pulling data from raw html. After reading up on the inner workings of a web crawler it became clear that I should be using Regular Expressions and quit fooling around with String functions such as indexOf and subString.

So basically, a regular expression is an ‘expression’ that is used to find patterns in text that are of interest to the user. I’m not going to go into specifics here as I am still learning the basics and don’t want to misinform anybody. If you’re interested you should visit the links below or if you’re already familiar with them and would like to see how to use them in C# read on.Here is a link to the MSDN documentation on the RegularExpression class.

The example below is a simplified version of the example on the MSDN site. It finds anchor tags in html and print the matches to the screen.

using System.Text.RegularExpressions;

String html = "<a href=\"http://www.conorhackett.com\">Conor Hackett</a>";

Regex expression = new Regex("<a\\s*[^<]+\\s*</a>");

// Run expression on html and store all matches in matches.
// MatchCollection stores all found matches in a collection of objects of type Match.
MatchCollection matches = expression.Matches(html);

// Print out all the found matches.
foreach (Match match in matches)
{
	Console.WriteLine(match.Value + "\n");
}

The above snippet should output

<a href="http://www.conorhackett.com">

Links:
http://www.ultrapico.com/Expresso.htm
http://www.codeproject.com/KB/dotnet/regextutorial.aspx
http://regexlib.com/