// http://stackoverflow.com/q/33707330/5290909
using System;
using System.Text.RegularExpressions;
 
public class Test
{
	public static void Main()
	{
		string strIn = @"<a href=""someurl.html?query=foo"">";
		string pattern = @"
			<
			  (?<Tag_Name>(a)|img)\b
			  [^>]*?
			  \b(?<URL_Type>(?(2)href|src))
			  \s*=\s*
			  (?:""(?<URL>(?>\\.|[^\\""_#?&]+)*(?:_|(?<Query>[#?&]))(?>\\.|[^""\\]+)*)""
			  |  '(?<URL>(?>\\.|[^\\'_#?&]+)*(?:_|(?<Query>[#?&]))(?>\\.|[^'\\]+)*)')
			";
		Regex re = new Regex( pattern,
								RegexOptions.IgnoreCase | RegexOptions.Multiline
								| RegexOptions.IgnorePatternWhitespace);
 
 
		MatchCollection underscoreLinks = re.Matches(strIn);
		for (int mnum = 0; mnum < underscoreLinks.Count; mnum++)
		{   //loop matches
		    Match match = underscoreLinks[mnum];
		    Console.WriteLine("Match #{0} - Value: {1}", mnum + 1, match.Value);
		}
	}
}