using System;
    using System.Text.RegularExpressions;
 
    public class Example
    {
       public static void Main()
       {
           string pattern =  @"\<A(G|H|I)\>\!([^\!]*)\!";
           string input = "<AI>!n!-Butyl acetate the quick brown <AI>!fox jumps! over the lazy dog!";
           string replacement = "$2";
           Regex rgx = new Regex(pattern);
           string result = rgx.Replace(input, replacement);
 
           Console.WriteLine("Original String:    '{0}'", input);
           Console.WriteLine("Replacement String: '{0}'", result);                             
        }
    }

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