diff --git a/src/hz_sophia.erl b/src/hz_sophia.erl
index e84e7a7..c73ef06 100644
--- a/src/hz_sophia.erl
+++ b/src/hz_sophia.erl
@@ -29,6 +29,11 @@ parse_literal2(Result, Tk, String) ->
 
 %%% Tokenizer
 
+-define(IS_ALPHA(C), ((((C) >= $A) and ((C) =< $Z)) or (((C) >= $a) and ((C) =< $z)) or ((C) == $_))).
+-define(IS_NUM(C), (((C) >= $0) and ((C) =< $9))).
+-define(IS_ALPHANUM(C), (?IS_ALPHA(C) or ?IS_NUM(C))).
+-define(IS_HEX(C), (?IS_NUM(C) or (((C) >= $A) and ((C) =< $F)) or (((C) >= $a) and ((C) =< $f)))).
+
 next_token({tk, Row, Col}, []) ->
     {ok, {{eof, "", Row, Col, Col}, {tk, Row, Col}, []}};
 next_token({tk, Row, Col}, " " ++ Rest) ->
@@ -41,46 +46,36 @@ next_token({tk, _, Col}, "\r" ++ Rest) ->
     next_token({tk, 1, Col + 1}, Rest);
 next_token({tk, _, Col}, "\n" ++ Rest) ->
     next_token({tk, 1, Col + 1}, Rest);
-next_token(Tk, [N | _] = String) when N >= $A, N =< $Z ->
+next_token(Tk, [C | _] = String) when ?IS_ALPHA(C) ->
     alphanum_token(Tk, Tk, String, []);
-next_token(Tk, [N | _] = String) when N >= $a, N =< $z ->
-    alphanum_token(Tk, Tk, String, []);
-next_token(Tk, "_" ++ _ = String) ->
-    alphanum_token(Tk, Tk, String, []);
-next_token(Tk, [N | _] = String) when N >= $0, N =< $9 ->
+next_token(Tk, [C | _] = String) when ?IS_NUM(C) ->
     num_token(Tk, Tk, String, []);
-next_token({tk, Row, Col}, "#" ++ Rest) ->
-    bytes_token({tk, Row, Col}, {tk, Row + 1, col}, Rest, "#");
+next_token({tk, Row, Col}, [$#, C | Rest]) when ?IS_HEX(C) ->
+    bytes_token({tk, Row, Col}, {tk, Row + 2, Col}, Rest, [C, $#]);
 next_token({tk, Row, Col}, [Char | Rest]) ->
     Token = {character, [Char], Row, Col, Col},
     {ok, {Token, {tk, Row + 1, Col}, Rest}}.
 
-alphanum_token(Start, {tk, Row, Col}, [C | Rest], Acc) when C >= $A, C =< $Z ->
+alphanum_token(Start, {tk, Row, Col}, [C | Rest], Acc) when ?IS_ALPHANUM(C) ->
     alphanum_token(Start, {tk, Row, Col}, Rest, [C | Acc]);
-alphanum_token(Start, {tk, Row, Col}, [C | Rest], Acc) when C >= $a, C =< $z ->
-    alphanum_token(Start, {tk, Row, Col}, Rest, [C | Acc]);
-alphanum_token(Start, {tk, Row, Col}, [C | Rest], Acc) when C >= $0, C =< $9 ->
-    alphanum_token(Start, {tk, Row, Col}, Rest, [C | Acc]);
-alphanum_token(Start, {tk, Row, Col}, [$_ | Rest], Acc) ->
-    alphanum_token(Start, {tk, Row, Col}, Rest, [$_ | Acc]);
 alphanum_token({tk, _, Start}, {tk, Row, End}, String, Acc) ->
     AlphaString = lists:reverse(Acc),
     Token = {alphanum, AlphaString, Row, Start, End},
     {ok, {Token, {tk, Row, End}, String}}.
 
-num_token(Start, {tk, Row, Col}, [N | Rest], Acc) when N >= $0, N =< $9 ->
-    num_token(Start, {tk, Row + 1, Col}, Rest, [N | Acc]);
+num_token(Start, {tk, Row, Col}, [C | Rest], Acc) when ?IS_NUM(C) ->
+    num_token(Start, {tk, Row + 1, Col}, Rest, [C | Acc]);
+num_token(Start, {tk, Row, Col}, [$_, C | Rest], Acc) when ?IS_NUM(C) ->
+    num_token(Start, {tk, Row + 2, Col}, Rest, [C, $_ | Acc]);
 num_token({tk, _, Start}, {tk, Row, End}, String, Acc) ->
     NumString = lists:reverse(Acc),
     Token = {integer, NumString, Row, Start, End},
     {ok, {Token, {tk, Row, End}, String}}.
 
-bytes_token(Start, {tk, Row, Col}, [N | Rest], Acc) when N >= $0, N =< $9 ->
-    bytes_token(Start, {tk, Row + 1, Col}, Rest, [N | Acc]);
-bytes_token(Start, {tk, Row, Col}, [N | Rest], Acc) when N >= $A, N =< $F ->
-    bytes_token(Start, {tk, Row + 1, Col}, Rest, [N | Acc]);
-bytes_token(Start, {tk, Row, Col}, [N | Rest], Acc) when N >= $a, N =< $f ->
-    bytes_token(Start, {tk, Row + 1, Col}, Rest, [N | Acc]);
+bytes_token(Start, {tk, Row, Col}, [C | Rest], Acc) when ?IS_HEX(C) ->
+    bytes_token(Start, {tk, Row + 1, Col}, Rest, [C | Acc]);
+bytes_token(Start, {tk, Row, Col}, [$_, C | Rest], Acc) when ?IS_HEX(C) ->
+    bytes_token(Start, {tk, Row + 1, Col}, Rest, [C, $_ | Acc]);
 bytes_token({tk, _, Start}, {tk, Row, End}, String, Acc) ->
     BytesString = lists:reverse(Acc),
     Token = {bytes, BytesString, Row, Start, End},
@@ -110,7 +105,7 @@ parse_expression(Type, Tk, String) ->
     parse_expression2(Type, NewTk, NewString, Token).
 
 parse_expression2(Type, Tk, String, {integer, S, Row, Start, End}) ->
-    Value = list_to_integer(S),
+    Value = convert_int(S),
     case Type of
         {_, _, integer} ->
             {ok, {Value, Tk, String}};
@@ -159,15 +154,38 @@ expect_tokens([Str | Rest], Tk, String) ->
             {error, {unexpected_token, Actual, Row, Start, End}}
     end.
 
+convert_int(Chars) ->
+    convert_int(Chars, 0).
+
+convert_int("_" ++ Chars, Result) ->
+    convert_int(Chars, Result);
+convert_int([N | Chars], Result) ->
+    Digit = N - $0,
+    NewResult = Result * 10 + Digit,
+    convert_int(Chars, NewResult);
+convert_int([], Result) ->
+    Result.
+
 convert_bytes(Chars) ->
-    Digits = lists:foldl(fun(C, Acc) -> [convert_nibble(C) | Acc] end, [], Chars),
+    % We do this as two reversing foldl type loops. One removes underscores and
+    % converts the ASCII into integers, and the other peels off pairs of
+    % numbers to form bytes.
+    Digits = reverse_convert_digits(Chars, []),
     reverse_combine_nibbles(Digits, <<>>).
 
-convert_nibble(C) when C >= $0, C =< $9 ->
+reverse_convert_digits("_" ++ Rest, Acc) ->
+    reverse_convert_digits(Rest, Acc);
+reverse_convert_digits([C | Rest], Acc) ->
+    Digit = convert_digit(C),
+    reverse_convert_digits(Rest, [Digit | Acc]);
+reverse_convert_digits([], Acc) ->
+    Acc.
+
+convert_digit(C) when C >= $0, C =< $9 ->
     C - $0;
-convert_nibble(C) when C >= $A, C =< $Z ->
+convert_digit(C) when C >= $A, C =< $Z ->
     C - $A + 10;
-convert_nibble(C) when C >= $a, C =< $z ->
+convert_digit(C) when C >= $a, C =< $z ->
     C - $a + 10.
 
 reverse_combine_nibbles([D1, D2 | Rest], Acc) ->
@@ -544,23 +562,23 @@ check_parser_with_typedef(Typedef, Sophia) ->
     % definitions in the AACI, so untyped parses probably don't work.
     check_sophia_to_fate(Type, Sophia, Fate).
 
-int_test() ->
-    check_parser("123").
+anon_types_test() ->
+    % Integers.
+    check_parser("123"),
+    check_parser("1_2_3"),
+    % Bytes.
+    check_parser("#DEAD000BEEF"),
+    check_parser("#DE_AD0_00B_EEF"),
+    % List of integers.
+    check_parser("[1, 2, 3]"),
+    % List of lists.
+    check_parser("[[], [1], [2, 3]]"),
+    % Tuple.
+    check_parser("(1, [2, 3], (4, 5))"),
+    % Map.
+    check_parser("{[1] = 2, [3] = 4}"),
 
-list_test() ->
-    check_parser("[1, 2, 3]").
-
-list_of_lists_test() ->
-    check_parser("[[], [1], [2, 3]]").
-
-tuple_test() ->
-    check_parser("(1, [2, 3], (4, 5))").
-
-maps_test() ->
-    check_parser("{[1] = 2, [3] = 4}").
-
-bytes_test() ->
-    check_parser("#DEAD000BEEF").
+    ok.
 
 records_test() ->
     TypeDef = "record pair = {x: int, y: int}",