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

public class Test
{
	public static void Main()
	{
    	var line = "this.Button.Value\nthis.value\ndocument.thisButton.Value";
    	var word = "this";
    	var rx =new Regex(string.Format(@"(?<=\s)\b{0}\b(?=\s)|\b{0}\b(?=\.)", word));
    	var result = rx.Replace(line, "NEW_WORD");
        Console.WriteLine(result);
	}
}