Extracting Groups and Sub-groups in RegEx

Question

This question is, in a way, continuation of my previously answered question: Getting "Unterminated [] set." Error in C#

I'm using regular expression in C# to extract URLs:

Regex find = new Regex(@"(?<First>[,""]url=)(?<Url>[^\\]+)(?<Last>\\u00)");

Where the text contains URLs in the format:

,url=http://domain.com?itag=25\u0026,url=http://hello.com?itag=11\u0026

I'm getting the entire URL in 'Url' group, but I'd also like to have the itag value in a separate "iTag" group. I know this can be done using sub-groups and I've been trying but can't figure out exactly how to do this.

Answer 1

You already have named groups defined in the Regex. The syntax ?<First> is naming everything within those parenthesis First.

When you match using Regex, using the Groups property to access the GroupCollection and extract a group value by name.

var first = regex.Match(line).Groups["First"].Value;

This will add an additional group for iTag, but retain the full Url. Move it outside the other parenthesis to change this.

(?<First>[,""]url=)(?<Url>[^\?]+?itag=(?<iTag>[0-9]*))(?<Last>\\u0026)

Here's the code.

Regex regex = new Regex("(?<First>[,\"]url=)(?<Url>[^\\?]*\\?itag=(?<iTag>[0-9]*))(?<Last>\\u0026)");
string input = ",url=http://domain.com?itag=25\u0026,url=http://hello.com?itag=11\u0026";

foreach(Match match in regex.Matches(input))
{
    System.Console.WriteLine("1. "+match);
    System.Console.WriteLine("  1. "+match.Groups["First"]);
    System.Console.WriteLine("  2. "+match.Groups["Url"]);
    System.Console.WriteLine("  3. "+match.Groups["iTag"]);
    System.Console.WriteLine("  4. "+match.Groups["Last"]);
}

Results:

1. ,url=http://domain.com?itag=25&
  1. ,url=
  2. http://domain.com?itag=25
  3. 25
  4. &
1. ,url=http://hello.com?itag=11&
  1. ,url=
  2. http://hello.com?itag=11
  3. 11
  4. &