\ExplSyntaxOn

% Useful regex strings
\regex_const:Nn \c__pos_int_regex { \d+ }

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%% Dice macro
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% Matches { # | #d# | #d# + # | #d# - # } while ignoring spaces
\regex_const:Nn \c__valid_dice_regex { \A\s*(\d+)(?:[dD](\d+)\s*(?:([+\-])\s*(\d+))?)?\s*\Z }

\seq_new:N \l__dice_args_seq
\tl_new:N \l__dice_number_tl
\tl_new:N \l__dice_sides_tl
\tl_new:N \l__dice_mod_sign_tl
\tl_new:N \l__dice_mod_tl

\msg_new:nnn { rpg } { dice / invalid_argument } { Invalid~argument~``#1''~passed~to~\noexpand\RpgDice. }

\NewDocumentCommand { \RpgDice } { m }
  {
    \group_begin:
      \regex_extract_once:NnNTF { \c__valid_dice_regex } { #1 } \l__dice_args_seq
        {
          \seq_pop_left:NN \l__dice_args_seq \l_tmpa_str % Don't need the full match
          \seq_pop_left:NN \l__dice_args_seq \l__dice_number_tl % # of dice
          \seq_pop_left:NN \l__dice_args_seq \l__dice_sides_tl % # of sides
          \seq_pop_left:NN \l__dice_args_seq \l__dice_mod_sign_tl % +/- for add/sub
          \seq_pop_left:NN \l__dice_args_seq \l__dice_mod_tl % modifier

          \str_if_empty:NTF { \l__dice_sides_tl } % are there dice ?
            { \l__dice_number_tl } % No, just a number
            { %Yes, break down the roll
              \fp_eval:n { floor ( \l__dice_number_tl * ( \l__dice_sides_tl + 1 ) / 2 )\l__dice_mod_sign_tl \l__dice_mod_tl }~

              (
                \l__dice_number_tl \dname \l__dice_sides_tl

                \str_case_e:nn { \l__dice_mod_sign_tl }
                  {
                    { + } { ~+~\l__dice_mod_tl }
                    { - } { ~--~\l__dice_mod_tl }
                  }
              )
            }
        }
        { \msg_error:nnn { rpg } { dice / invalid_argument } { #1 }}
    \group_end:
  }
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% Gets the width of a space
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\box_new:N \l__rpg_space_box
\hbox_set:Nn \l__rpg_space_box {~}
\dim_new:N \l__rpg_space_dim
\dim_set:Nn \l__rpg_space_dim { \box_wd:N \l__rpg_space_box }

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% Some keys are required for proper execution of macros.
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\msg_new:nnn { rpg } { key_required } { #1~requires~key~``#2''~to~be~set. }

% #1 - key value
% #2 - macro name
% #3 - key name
\cs_new_protected:Npn \__rpg_check_for_key:Nnn #1#2#3
  {
    \group_begin:
      \str_set:Nx \l_tmpa_str { #1 }

      \str_if_empty:NT { \l_tmpa_str }
        { \msg_error:nnnn { rpg } { key_required } { #2 } { #3 } }
    \group_end:
  }