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function Replace(S, Old, New: String): TStringList;
var
  i: Integer;
  T: String;
begin
  Result:=TStringList.Create;

  T:=StringReplace(S,New,'(Old)',[rfReplaceAll,rfIgnoreCase]);
  T:=StringReplace(T,'(Old)',Old,[rfReplaceAll,rfIgnoreCase]);
  Result.Add(T);

  T:=StringReplace(S,Old,'(New)',[rfReplaceAll,rfIgnoreCase]);
  T:=StringReplace(T,New,'(Old)',[rfReplaceAll,rfIgnoreCase]);
  T:=StringReplace(T,'(New)',New,[rfReplaceAll,rfIgnoreCase]);
  T:=StringReplace(T,'(Old)',Old,[rfReplaceAll,rfIgnoreCase]);
  Result.Add(T);

  T:=StringReplace(T,Old,'(New)',[rfReplaceAll,rfIgnoreCase]);
  T:=StringReplace(T,New,'(Old)',[rfReplaceAll,rfIgnoreCase]);
  T:=StringReplace(T,'(New)',New,[rfReplaceAll,rfIgnoreCase]);
  T:=StringReplace(T,'(Old)',Old,[rfReplaceAll,rfIgnoreCase]);
  Result.Add(T);

  T:=StringReplace(S,Old,'(New)',[rfReplaceAll,rfIgnoreCase]);
  T:=StringReplace(T,'(New)',New,[rfReplaceAll,rfIgnoreCase]);
  Result.Add(T);
end;					

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procedure TForm1.Button1Click(Sender: TObject);
var
  t: TStringList;
begin
  t:=TStringList.Create;
  t:=Replace('ABCD','B','C');
  Memo1.Lines.Assign(t);
  t.Free;
end;