using System;
using System.Text;
using System.Text.RegularExpressions;
 
class M
{
    static void Main(string[] args)
    {
        string[] x =new[]{ @"asoetuh T.C.A. satoeuhs TaCaA",
        @"T.C.A. basoetuhase",
        @"aoethT.C.A.tnetnn"
        };
 
        string[] y = new[] {
          @"asoetu_T.C.A.aoesuth", "_T.C.A. satoheustn", "satoheu _T.C.A.", "aoeu TbCaA"
        };
 
        string repl = "%%%%";
 
        foreach (var item in x)
        {
            Console.WriteLine("{0} -> {1}", item, Regex.Replace(item, @"(?<!_)" + Regex.Escape(@"T.C.A."), repl));
        }
 
        foreach (var item in y)
        {
            Console.WriteLine("{0} -> {1}", item, Regex.Replace(item, @"(?<!_)" + Regex.Escape(@"T.C.A."), repl));
        }
    }
}

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