using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Text.RegularExpressions;

public class Test
{
	public static void Main()
	{
		var s = "wordA wordB wordC wordB \n wordD";
    	var pattern = @"(?<=wordA.*?)wordB(?=.*?wordD)";
    	var result = Regex.Replace(s, pattern, "<<<$&>>>", RegexOptions.Singleline);
      	Console.WriteLine(result);
	}
}