using System;
using System.Text.RegularExpressions;
public class Test
{
	public static void Main()
	{
		MatchThis("H12345678ABC");
		MatchThis("G123456789ABC");
	}
	private static void MatchThis(string subject)
	{
		var regex = new Regex(@"(?((?<hgroup>H))\k<hgroup>.{8}|.{10})(?<user>.*)");
		var match =regex.Match(subject);
		if(match.Success)
		{
		    Console.WriteLine(match.Groups["user"].Value);//prints ABC
		}
		else
		{
			Console.WriteLine("No Match");
		}
	}
}

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