using System;

using System.Text.RegularExpressions;

public class Program

{

    public static void Main()

    {

        string fixThis = @"Hans4444müller,Mary555kren";

​

        var re = new Regex("\\d+");

        //Replace every match, the function 'GetRomanText' is invoked to create the replacement value for each hit:

        var newText = re.Replace(fixThis, GetRomanText);

        Console.WriteLine(newText);

    }

​

    private static string GetRomanText(Match m) {

        string[] roman = new[] { "I", "II", "III", "IV", "V", "VI", "VII", "VIII", "IX", "IX" };

        string result = "";

        //Get the ASCII value of the first character in the match, 

        char c = m.Value[0];

        //It should be a number, test for safety:

        if (c >= 48 && c <= 57) {

            int index = c - 48;

            result = roman[index];

        }

​

        return result;

    }

}