using System;
using System.Text.RegularExpressions;
 
public class Test
{
	public static void Main()
	{
		var input = "CAAAAACCCGGAAAAACGGCTTAGCTAGCTGGATCGAGGATAGCTGGATAGCTAAAAATAGTCGATAATCGAAATCGAGAGACTAAAGAAATCA";
        var codon = "AAA";
        var replacement = "ACC";
 
        var replaced = Regex.Replace(input, @"\G((?:.{3})*?)" + codon, "$1" + replacement);
 
        Console.WriteLine(input);
        Console.WriteLine(replaced, @"\G.{3}", "$0 ");
 
        Console.WriteLine(Regex.Replace(input, @"\G.{3}", "$0 "));
        Console.WriteLine(Regex.Replace(replaced, @"\G.{3}", "$0 "));
	}
}

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