Reverse a number before treating this as a big number.

reverse.erl

-module(reverse).
-export([reverse/1]).

%% @doc reverse a number
-spec(reverse(string()) -> string()).
reverse(ANumber) ->
	%% reverse the string
	Nmbr = string:reverse(ANumber),
	%% verify if is not a number
	Check = not_a_number(Nmbr, 0, true),

	if 
		Check == true -> "error";
		Check == false -> Nmbr
	end.


%% @doc verify if it's a number
-spec(not_a_number(string(), number(), atom()) -> atom()).
not_a_number(Nmbr, Index, Result) ->
	Result = list:contains(string:substr(Nmbr, Index, Index), [1,2,3,4,5,6,7,8,9]),
	Index = Index + 1,
	Length = string:len(Nmbr),
	
	if
		Result == false -> false;
		Index <= Length -> not_a_number(Nmbr, Index, Result);
		Index > Length -> true
	end.

%% regex to check if it'a number or a recursion checking every possibility