using System;
using System.Linq;
using System.Text.RegularExpressions;
 
public class Test
{
	public static void Main()
	{
		var lineOfKeys = "SiteElements[10].TempateElementId,SiteElements[10].TemplateElementValue,SiteElements[11].TempateElementId,SiteElements[11].TemplateElementValue";
		var uniqueSiteElementKeys3 = Regex.Matches(lineOfKeys, @"\bSiteElements\[[0-9]+\]")
		                                  .Cast<Match>()
		                                  .Select(match => match.Value)
		                                  .Distinct()
		                                  .ToList();
 
        foreach (var item in uniqueSiteElementKeys3)
        	Console.WriteLine(item);
	}
}

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