Заменяем текст в текстовом файле

procedure FileReplaceString(const FileName, searchstring, replacestring: string); 
var 
  fs: TFileStream; 
  S: string; 
begin 
  fs := TFileStream.Create(FileName, fmOpenread or fmShareDenyNone); 
  try 
    SetLength(S, fs.Size); 
    fs.ReadBuffer(S[1], fs.Size); 
  finally 
    fs.Free; 
  end; 
  S  := StringReplace(S, SearchString, replaceString, [rfReplaceAll, rfIgnoreCase]); 
  fs := TFileStream.Create(FileName, fmCreate); 
  try 
    fs.WriteBuffer(S[1], Length(S)); 
  finally 
    fs.Free; 
  end; 
end;