using System;

using System.Collections.Generic;

using System.Text.RegularExpressions;

using System.Linq;

public class Program

{

    public static void Main()

    {

        var substitutions = new Dictionary<string, string>()

        {

            { "one", "two" },

            { "two", "three" },

        };

​

        var phrase = "one two three four gone";

​

        var pattern = string.Join("|", substitutions.Keys.Select(x => @$"\b{Regex.Escape(x)}\b"));

​

        // Pattern is: \bone\b|\btwo\b

        var result = Regex.Replace(phrase, pattern, match => substitutions[match.Value]);

        Console.WriteLine(result);

    }

}