Regular expression keyword marking

Question

Good morning! I have been two weeks evolving in the treatment of texts in Sqlserver (through CLR) with the use of regular expressions. Now my challenge is to "mark" names of municipalities so that they are not "treated". It would be like a list of stopwors, but in reverse. My idea is to put the "_" sign at the beginning and end of each county name. Using the code:

using System;
using System.Text.RegularExpressions;

public class Exemplo
{
    public static void Main()
    {
        string pattern = @"(\bRio de Janeiro\b|\bManaus\b)";
        string[] titles = { "As cidades de Manaus e Rio de Janeiro" };
        string replacement = "_$1_";
        foreach (string title in titles)
            Console.WriteLine(Regex.Replace(title, pattern, replacement));
    }
}

would result in: "The cities of _Manaus_ and _Rio de Janeiro_"

How, in a performative way, I could have the result: "The cities of _Manaus_ and _Rio_de_janeiro_"?

Or another way to ignore a word relationship while processing text.

With the replacement of the internal spaces by _, In a future treatment (I’m using a stemmer) after the removal of the underline, the result would be "rio de janeiro" and "Manaus" and not "rio dar Jane" and "man", what happens to my code currently.

Grateful