using System;
using System.IO;
using System.Text.RegularExpressions;
 
public class Test
{
	public static void Main()
	{
 
		var Value = "usual text1<tagID11111111>text without end tag<tagID22222222>formated text1<standarttag>usual text2<tagID33333333>formated text2<standarttag>usual text3";
		var splits = Regex.Split(Value, @"((?:<tagID\d+>)?[^<]+(?:<standarttag>)?)");
		foreach (var s in splits)
		    if (!string.IsNullOrEmpty(s))
		        Console.WriteLine(s);
	}
}

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