using System;

using System.Text.RegularExpressions;

public class Program

{

    public void Main()

    {

        string text = "A<sub>hello</sub>B";

        string subRegex = "(?<=<sub>)(?<content>.+)(?=</sub>)";

        var x = F(text, subRegex, "content", (g) => " " + g + " ");

        Console.WriteLine(x);

    }

    public string F(string input, string regex, string group, Func<string, string> f)

    {

        Regex subRegex = new Regex(regex, RegexOptions.IgnoreCase);

        return subRegex.Replace(input, m =>

        {

            var gr = m.Groups[group].Value;

            return f(gr);

        });

    }

}