// antes
function RemoverNaoNumericos(const S: string): string;
var
  DIdx: Integer;
  SIdx: Integer;
begin
  SetLength(Result, Length(S));
  DIdx := 0;
  for SIdx := 1 to Length(S) do
    if S[SIdx] in ['0'..'9'] then
    begin
      Inc(DIdx);
      Result[DIdx] := S[SIdx]
    end;
  SetLength(Result, DIdx)
end;

// hoje

function TLibSGNStrings.RemoverNaoNumericos(ATexto: string): string;
var
  C:  Char;
begin
  Result := EmptyStr;
  for C in ATexto do
    Result := Result + IfThen(C.IsDigit,C);
end;