Soundex algorithm in Erlang

soundex.erl

#!/usr/bin/env escript
%% -*- erlang -*-
%%! -smp enable -sname soundex -mnesia debug verbose

main([String]) ->
    Word = soundex(String),
    io:format("~s => ~s\n", [String, Word]);
main(_) ->
    usage().

usage() ->
    io:format("usage: soundex string\n"),
    halt(1).

soundex(L) when is_list(L) ->
    soundex(L, 4).

soundex([H|T], Length) ->
    Len = Length - 1,
    if
        H >= $A, H =< $Z ->
            [H|soundex1(T, Len)];
        H >= $a, H =< $z ->
            N = H - ($a - $A),
            [N|soundex1(T, Len)];
        true ->
            soundex(T)
    end;
soundex([], _) ->
    [].

soundex1(L, Length) when is_list(L) ->
    Result = soundex1(L, "?", Length),
    Diff = Length - length(Result),
    if
        Diff > 0 ->
            Result ++ lists:duplicate(Diff, $0);
        true ->
            Result
    end.

soundex1([H|T], PrevChar, Length) when Length > 0 ->
    if
        H >= $A, H =< $Z ->
            SoundexMap = #{
                $A => $0,
                $B => $1,
                $C => $2,
                $D => $3,
                $E => $0,
                $F => $1,
                $G => $2,
                $H => $0,
                $I => $0,
                $J => $2,
                $K => $2,
                $L => $4,
                $M => $5,
                $N => $5,
                $O => $0,
                $P => $1,
                $Q => $2,
                $R => $6,
                $S => $2,
                $T => $3,
                $U => $0,
                $V => $1,
                $W => $0,
                $X => $2,
                $Y => $0,
                $Z => $2
            },
            Char = maps:get(H, SoundexMap),
            if
                Char =/= PrevChar, Char =/= $0 ->
                    [Char|soundex1(T, Char, Length-1)];
                true ->
                    soundex1(T, PrevChar, Length)
            end;
        H >= $a, H =< $z ->
            N = H - ($a - $A),
            soundex1([N|T], PrevChar, Length);
        true ->
            soundex1(T, PrevChar, Length)
    end;
soundex1(_, _, Length) when Length =< 0 ->
    [];
soundex1([], _, _) ->
    [].