Here's a little challenge. The goal is to create a command that takes is two or three arguments without using square brackets for optional arguments. The first and second argument are separated by a comma and a space, while the second and the third (if present) are separated by a dash. This means that both of the following inputs should be acceptable

mycommand{arg_1, arg_2}

and

mycommand{arg_1, arg_2-arg_3}

The definition of what mycommand does with the two/three arguments is irrelevant to the problem.

You can do it with xparse:

documentclass{article}

usepackage{xparse}



% split at the comma

NewDocumentCommand{mycommand}{ >{SplitArgument{1}{,}}m }{%

  mycommandA#1%

}

% do something with #1 and split the second part at the hyphen

NewDocumentCommand{mycommandA}{ m >{SplitArgument{1}{-}}m }{%

  Main is #1%

  IfNoValueTF{#2}{.}{; mycommandB#2}%

}

% do something with the second part

NewDocumentCommand{mycommandB}{mm}{%

  Secondary is #1IfValueT{#2}{ plus #2}.%

}



begin{document}



mycommand{arg1}



mycommand{arg1, arg2}



mycommand{arg1, arg2-arg3}



end{document}

An advantage: it's irrelevant if you forget the space after the comma.

documentclass{article}



defmycommand#1{xmycommand#1--relax}

defxmycommand#1,#2-#3-#4relax{[#1][#2][#3]}





begin{document}



$mycommand{arg_1, arg_2}$



$mycommand{arg_1, arg_2-arg3}$



end{document}

A listofitems approach.

documentclass{article}

usepackage{listofitems}

newcommandmycommand[1]{setsepchar{,/-}readlist*myargs{#1}%

  noindent Arg 1 is myargs[1]\

  Arg 2 is myargs[2,1]\

  Arg 3 is ifnumlistlenmyargs[2]=1relax Absentelsemyargs[2,2]fi%

}

begin{document}



mycommand{arg1, arg2}



and



mycommand{arg1, arg2-arg3}



end{document}

Some crocheting/knitting by hand (or whatever)... ;-)

I suppose this question came into being while you thought about your other question: Custom cite command. In that question you ask for a customized cite-command with the same syntax.

Be that as it may.

The challenge is doing it in a way where desired space-removal but no undesired removal of argument-braces takes place.

E.g., with things like mycommand{{arg_1}a, arg_2} the braces surrounding arg_1 should probably be preserved while with things like mycommand{{arg_1} , arg_2} they should probably be removed.

Also probably only space-tokens surrounding the entire arguments/surrounding those commas and those dashes that are taken for argument-separators should get removed.

Below is an approach where comma takes precedence over the dash where and the leftmost unhidden comma delimits the first argument from the remaining arguments and where the first unhidden dash following that comma delimits the second from the third argument.

Exactly one level of braces will be removed in case after removing surrounding spaces it surrounds an entire argument as in that case it is assumed that the braces serve for hiding commas/dashes/spaces/emptiness. In all other cases braces will be preserved.

E.g., with things like mycommand{ {arg_1} , {arg_2-2} - arg_3} you should get:

First argument: arg_1

Second argument: arg_2-2

Third argument: arg_3.

E.g., with things like mycommand{ {{arg 1,a} - arg 1b} , {arg 2-2} , 2 - arg 3a - arg 3b} you should get:

First argument (left from the first unhidden comma, surrounding spaces and outermost brace-level that surrounds the entire argument removed): {arg 1,a} - arg 1b

Second argument (right from the first unhidden comma, left from the first unhidden dash following the comma): {arg 2-2} , 2

Third argument (the remainder, right from the first unhidden dash following the comma): arg 3a - arg 3b.

E.g., with things like mycommand{{arg 1}, {arg 2-2} , 2 - arg 3a - arg 3b} you should get:

First argument (left from the first unhidden comma): arg 1

Second argument (right from the first unhidden comma, left from the first unhidden dash following the comma): {arg 2-2} , 2

Third argument (the remainder, right from the first unhidden dash following the comma): arg 3a - arg 3b.

You may not be familiar to methods for removing space-tokens that surround arguments, thus a quote from the manual of my deprecated labelcas-package:

A note about removing leading and trailing spaces

The matter of removing trailing spaces from an (almost) arbitrary
token-sequence is elaborated in detail by Michael Downes, 'Around the
Bend #15, answers', a summary of internet-discussion which took place
under his guidance primarily at the INFO-TEX list, but also at
comp.text.tex (usenet) and via private email; December 1993. Online
archived at http://www.tug.org/tex-archive/info/arobend/answer.015.

One basic approach suggested therein is using TeX's scanning of
delimited parameters in order to detect and discard the ending space
of an argument:

... scan for a pair of tokens: a space-token and some well-chosen
bizarre token that can't possibly occur in the scanned text. If you
put the bizarre token at the end of the text, and if the text has a
trailing space, then TeX's delimiter matching will match at that point
and not before, because the earlier occurrences of space don’t have
the requisite other member of the pair.

Next consider the possibility that the trailing space is absent: TeX
will keep on scanning ahead for the pair
⟨space⟩⟨bizarre⟩
until either it finds them or it decides to give up and signal a
'Runaway argument?' error. So you must add a stop pair to catch the
runaway argument possibility: a second instance of the bizarre
token, preceded by a space. If TeX doesn't find a match at the
first bizarre token, it will at the second one.

(Look up the macros KV@@sp@def, KV@@sp@b, KV@@sp@c and KV@@sp@d in
David Carlisle’s keyval-package for an interesting variation on this
approach.)

When scanning for parameters
##1⟨space⟩⟨bizarre⟩##2⟨B1⟩

the sequence:
⟨stuff where to remove trail-space⟩⟨bizarre⟩⟨space⟩⟨bizarre⟩⟨B1⟩

, you can fork two cases:

1. Trailing-space:
   ##1 = ⟨stuff where to remove trail-space⟩, but with removed space. (And possibly one
   removed brace-level!)
   ##2 = ⟨space⟩⟨bizarre⟩.




2. No trailing-space:
   ##1 = ⟨stuff where to remove trail-space⟩⟨bizarre⟩.
   ##2 is empty.

So forking can be implemented depending on the emptiness of ##2.

You can easily prevent the brace-removal in the first case, e.g., by adding (and
later removing) something (e.g., a space-token) in front of the
⟨stuff where to remove trail-space⟩.

You can choose ⟨B1⟩=⟨bizarre⟩⟨space⟩.

The link in the quote is out of date.

Nowadays you can find the entire "Around the bend" -collection at http://mirrors.ctan.org/info/challenges/AroBend/AroundTheBend.pdf.

As you see, the methods for space-removal exhibited in the quote above do rely on some sequence of tokens that must not occur within the argument.

In the example below it is relied on the token UD@seldom not occurring within arguments.

In other words: You must not use the token UD@seldom within your arguments.

If you don't like that restriction, then I can deliver space-removal-routines which do without such restrictions, but they are slower and they form another huge load of code.

documentclass{article}



makeatletter

%%-----------------------------------------------------------------------------

%% Paraphernalia ;-) :

%%.............................................................................

newcommandUD@firstoftwo[2]{#1}%

newcommandUD@secondoftwo[2]{#2}%

newcommandUD@exchange[2]{#2#1}%

newcommandUD@removespace{}UD@firstoftwo{defUD@removespace}{} {}%

%%-----------------------------------------------------------------------------

%% Check whether argument is empty:

%%.............................................................................

%% UD@CheckWhetherNull{<Argument which is to be checked>}%

%%                     {<Tokens to be delivered in case that argument

%%                       which is to be checked is empty>}%

%%                     {<Tokens to be delivered in case that argument

%%                       which is to be checked is not empty>}%

%% The gist of this macro comes from Robert R. Schneck's ifempty-macro:

%% <https://groups.google.com/forum/#!original/comp.text.tex/kuOEIQIrElc/lUg37FmhA74J>

%%.............................................................................

newcommandUD@CheckWhetherNull[1]{%

  romannumeral0expandafterUD@secondoftwostring{expandafter

  UD@secondoftwoexpandafter{expandafter{string#1}expandafter

  UD@secondoftwostring}expandafterUD@firstoftwoexpandafter{expandafter

  UD@secondoftwostring}expandafterexpandafterUD@firstoftwo{ }{}%

  UD@secondoftwo}{expandafterexpandafterUD@firstoftwo{ }{}UD@firstoftwo}%

}%

%%-----------------------------------------------------------------------------

%% Check whether argument's first token is a catcode-1-character

%%.............................................................................

%% UD@CheckWhetherBrace{<Argument which is to be checked>}%

%%                      {<Tokens to be delivered in case that argument

%%                        which is to be checked has leading

%%                        catcode-1-token>}%

%%                      {<Tokens to be delivered in case that argument

%%                        which is to be checked has no leading

%%                        catcode-1-token>}%

newcommandUD@CheckWhetherBrace[1]{%

  romannumeral0expandafterUD@secondoftwoexpandafter{expandafter{%

  string#1.}expandafterUD@firstoftwoexpandafter{expandafter

  UD@secondoftwostring}expandafterexpandafterUD@firstoftwo{ }{}%

  UD@firstoftwo}{expandafterexpandafterUD@firstoftwo{ }{}UD@secondoftwo}%

}%

%%-----------------------------------------------------------------------------

%% Check whether brace-balanced argument starts with a space-token

%%.............................................................................

%% UD@CheckWhetherLeadingSpace{<Argument which is to be checked>}%

%%                             {<Tokens to be delivered in case <argument

%%                               which is to be checked>'s 1st token is a

%%                               space-token>}%

%%                             {<Tokens to be delivered in case <argument

%%                               which is to be checked>'s 1st token is not

%%                               a space-token>}%

newcommandUD@CheckWhetherLeadingSpace[1]{%

  romannumeral0UD@CheckWhetherNull{#1}%

  {expandafterexpandafterUD@firstoftwo{ }{}UD@secondoftwo}%

  {expandafterUD@secondoftwostring{UD@CheckWhetherLeadingSpaceB.#1 }{}}%

}%

newcommandUD@CheckWhetherLeadingSpaceB{}%

longdefUD@CheckWhetherLeadingSpaceB#1 {%

  expandafterUD@CheckWhetherNullexpandafter{UD@secondoftwo#1{}}%

  {UD@exchange{UD@firstoftwo}}{UD@exchange{UD@secondoftwo}}%

  {UD@exchange{ }{expandafterexpandafterexpandafterexpandafter

   expandafterexpandafterexpandafter}expandafterexpandafter

   expandafter}expandafterUD@secondoftwoexpandafter{string}%

}%

%%-----------------------------------------------------------------------------

%% Extract first inner undelimited argument:

%%.............................................................................

%%   UD@ExtractFirstArg{ABCDE} yields  {A}

%%

%%   UD@ExtractFirstArg{{AB}CDE} yields  {AB}

%%.............................................................................

newcommandUD@RemoveTillUD@SelDOm{}%

longdefUD@RemoveTillUD@SelDOm#1#2UD@SelDOm{{#1}}%

newcommandUD@ExtractFirstArg[1]{%

  romannumeral0%

  UD@ExtractFirstArgLoop{#1UD@SelDOm}%

}%

newcommandUD@ExtractFirstArgLoop[1]{%

  expandafterUD@CheckWhetherNullexpandafter{UD@firstoftwo{}#1}%

  { #1}%

  {expandafterUD@ExtractFirstArgLoopexpandafter{UD@RemoveTillUD@SelDOm#1}}%

}%

%%-----------------------------------------------------------------------------

%% UD@RemoveSpaces{<argument>} removes leading and trailing spaces from 

%% <argument>.

%% !!!! <argument> must not contain the token UD@seldom !!!!

%%.............................................................................

begingroup

newcommandUD@RemoveSpacesAndOneLevelOfBraces[1]{%

  endgroup

  newcommandUD@RemoveSpacesAndOneLevelOfBraces[1]{%

    romannumeral0%

    UD@trimtrailspaceloop#1##1UD@seldom#1UD@seldomUD@seldom#1{##1}%

  }%

  newcommandUD@trimtrailspaceloop{}%

  longdefUD@trimtrailspaceloop##1#1UD@seldom##2UD@seldom#1##3{%

    UD@CheckWhetherNull{##2}{%

      UD@trimleadspaceloop{##3}%

    }{%

      UD@trimtrailspaceloop##1UD@seldom#1UD@seldomUD@seldom#1{##1}%

    }%

  }%

}%

UD@RemoveSpacesAndOneLevelOfBraces{ }%

newcommandUD@trimleadspaceloop[1]{%

  UD@CheckWhetherLeadingSpace{#1}{%

    expandafterUD@trimleadspaceloopexpandafter{UD@removespace#1}%

  }{%

    UD@CheckWhetherNull{#1}{ }{%

      expandafterUD@CheckWhetherNullexpandafter{UD@firstoftwo{}#1}{%

        UD@exchange{ }{expandafter}UD@secondoftwo{}#1%

      }{ #1}%

    }%

  }%

}%

%%-----------------------------------------------------------------------------

%% Check whether argument contains no comma which is not nested 

%% in braces:

%%.............................................................................

%% UD@CheckWhetherNoComma{<Argument which is to be checked>}%

%%                        {<Tokens to be delivered in case that argument

%%                          contains no comma>}%

%%                        {<Tokens to be delivered in case that argument

%%                          contains comma>}%

%%

newcommandUD@GobbleToComma{}longdefUD@GobbleToComma#1,{}%

newcommandUD@CheckWhetherNoComma[1]{%

  expandafterUD@CheckWhetherNullexpandafter{UD@GobbleToComma#1,}%

}%

%%-----------------------------------------------------------------------------

%% Check whether argument contains no dash which is not nested 

%% in braces:

%%.............................................................................

%% UD@CheckWhetherNoDash{<Argument which is to be checked>}%

%%                       {<Tokens to be delivered in case that argument

%%                         contains no dash>}%

%%                       {<Tokens to be delivered in case that argument

%%                         contains dash>}%

%%

newcommandUD@GobbleToDash{}longdefUD@GobbleToDash#1-{}%

newcommandUD@CheckWhetherNoDash[1]{%

  expandafterUD@CheckWhetherNullexpandafter{UD@GobbleToDash#1-}%

}%

%%-----------------------------------------------------------------------------

%% Take a comma-delimited/dash-delimited argument where a space was

%% prepended for preventing brace-removal and wrap it in curly braces.

%%.............................................................................    

newcommandUD@SplitCommaArg{}%

longdefUD@SplitCommaArg#1,{%

  romannumeral0UD@exchange{ }{expandafter}expandafter{UD@removespace#1}%

}%

newcommandUD@SplitDashArg{}%

longdefUD@SplitDashArg#1-{%

  romannumeral0UD@exchange{ }{expandafter}expandafter{UD@removespace#1}%

}%

%%-----------------------------------------------------------------------------

%% Now we have the tools for creating the desired mycommand-macro:

%%.............................................................................    

newcommandmycommand[1]{%

  romannumeral0%

  UD@CheckWhetherNoComma{#1}{%

    expandafterUD@exchangeexpandafter{%

       expandafter{%

         romannumeral0UD@exchange{ }{expandafterexpandafterexpandafter}%

         UD@RemoveSpacesAndOneLevelOfBraces{#1}%

       }%

    }{mycommand@oneargument}%

  }{%

    expandafterUD@exchangeexpandafter{expandafter%

      {%

      romannumeral0%

      UD@exchange{ }{expandafterexpandafterexpandafter}%

      expandafterUD@firstoftwoexpandafter{expandafter}%

      romannumeral0%

      UD@exchange{ }{%

        expandafterexpandafterexpandafterexpandafter

        expandafterexpandafterexpandafter

      }%

      expandafterUD@SplitCommaArg

      UD@firstoftwo{ }{}#1}%

    }{%

      expandafter@mycommand

      romannumeral0%

      UD@exchange{ }{%

        expandafterexpandafterexpandafterexpandafter

        expandafterexpandafterexpandafter

      }%

      expandafterUD@ExtractFirstArgexpandafter{%

      romannumeral0%

      UD@exchange{ }{%

        expandafterexpandafterexpandafterexpandafter

        expandafterexpandafterexpandafter

      }%

      expandafterUD@SplitCommaArg

      UD@firstoftwo{ }{}#1}%

    }%

  }%

}%

newcommand@mycommand[2]{%

  UD@CheckWhetherNoDash{#2}{%

    expandafterUD@exchangeexpandafter{expandafter

      {%

        romannumeral0%

        UD@exchange{ }{expandafterexpandafterexpandafter}%

        UD@RemoveSpacesAndOneLevelOfBraces{#2}%

      }%

    }{%

      expandafterUD@exchangeexpandafter{%

        expandafter{%

          romannumeral0%

          UD@exchange{ }{expandafterexpandafterexpandafter}%

          UD@RemoveSpacesAndOneLevelOfBraces{#1}%

        }%

      }{mycommand@twoarguments}%

    }%

  }{%

    expandafterUD@exchangeexpandafter{expandafter%

      {%

      romannumeral0%

      UD@exchange{ }{expandafterexpandafterexpandafter}%

      expandafterUD@firstoftwoexpandafter{expandafter}%

      romannumeral0%

      UD@exchange{ }{%

        expandafterexpandafterexpandafterexpandafter

        expandafterexpandafterexpandafter

      }%

      expandafterUD@SplitDashArg

      UD@firstoftwo{ }{}#2}%

    }{%

      expandafter@@mycommand

      romannumeral0%

      UD@exchange{ }{%

        expandafterexpandafterexpandafterexpandafter

        expandafterexpandafterexpandafter

      }%

      expandafterUD@ExtractFirstArgexpandafter{%

      romannumeral0%

      UD@exchange{ }{%

        expandafterexpandafterexpandafterexpandafter

        expandafterexpandafterexpandafter

      }%

      expandafterUD@SplitDashArg

      UD@firstoftwo{ }{}#2}%

    }%

    {#1}%

  }%

}%

newcommand@@mycommand[3]{%

  expandafterUD@exchangeexpandafter{%

    expandafter{%

      romannumeral0%

      UD@exchange{ }{expandafterexpandafterexpandafter}%

      UD@RemoveSpacesAndOneLevelOfBraces{#2}%

    }%

  }{%

    expandafterUD@exchangeexpandafter{%

       expandafter{%

         romannumeral0%

         UD@exchange{ }{expandafterexpandafterexpandafter}%

         UD@RemoveSpacesAndOneLevelOfBraces{#1}%

      }%

    }{%

      expandafterUD@exchangeexpandafter{%

         expandafter{%

           romannumeral0%

           UD@exchange{ }{expandafterexpandafterexpandafter}%

           UD@RemoveSpacesAndOneLevelOfBraces{#3}%

        }%

      }{mycommand@threearguments}%

    }%

  }%

}%

newcommandmycommand@oneargument[1]{%

  UD@secondoftwo{%

    %We need a space to stop romannumeral-expansion.

  }{ }%

  There is no comma or only empty args after the comma, thus:\

  argument 1 = printmeaning{#1}%

}%

newcommandmycommand@twoarguments[2]{%

  UD@CheckWhetherNull{#2}{%

    mycommand@oneargument{#1}%

  }{%

    UD@secondoftwo{%

      %We need a space to stop romannumeral-expansion.

    }{ }%

    There is a comma but either no dash or only one non-empty argument near

    the dash, thus:\

    argument 1 = printmeaning{#1}\

    argument 2 = printmeaning{#2}%

  }%

}%

newcommandmycommand@threearguments[3]{%

  UD@CheckWhetherNull{#2}{%

    UD@CheckWhetherNull{#3}{%

      mycommand@oneargument{#1}%

    }{%x

      mycommand@twoarguments{#1}{#3}%

    }%

  }{%

    UD@CheckWhetherNull{#3}{%

      mycommand@twoarguments{#1}{#2}%

    }{%

      UD@secondoftwo{%

        %We need a space to stop romannumeral-expansion.

      }{ }%

      There is a comma and a dash, thus:\

      argument 1 = printmeaning{#1}\

      argument 2 = printmeaning{#2}\

      argument 3 = printmeaning{#3}%

    }%

  }%

}%



newcommandprintmeaning[1]{%

  defUD@tempa{#1}%

  fbox{texttt{expandafterstrip@prefixmeaningUD@tempa}}%

}%



makeatother



parindent=0ex

parskip=smallskipamount



begin{document}



verb|mycommand{  arg 1  }|:\

mycommand{  arg 1  }\

nullhrulefillnull



verb|mycommand{ arg 1 , }|:\

mycommand{ arg 1 , }\

nullhrulefillnull    



verb|mycommand{ arg 1 ,  - }|:\

mycommand{ arg 1 ,  - }\

nullhrulefillnull



verb|mycommand{ arg 1 , arg 2 - }|:\

mycommand{ arg 1 , arg 2 - }\

nullhrulefillnull



verb|mycommand{ arg 1 , arg 2 - arg 3}|:\

mycommand{ arg 1 , arg 2 - arg 3}\

nullhrulefillnull



verb|mycommand{ arg 1 , - arg 3}|:\

mycommand{ arg 1 , - arg 3}\

nullhrulefillnull



verb|mycommand{  {arg 1a} - arg 1b ,  {arg 2-2} , 2 -  arg 3a - arg 3b}|:\

mycommand{  {arg 1a} - arg 1b ,  {arg 2-2} , 2 -  arg 3a - arg 3b}\

nullhrulefillnull



verb|mycommand{ {arg 1} ,  {arg 2-2} , 2 -  arg 3a - arg 3b}|:\

mycommand{ {arg 1} ,  {arg 2-2} , 2 -  arg 3a - arg 3b}



end{document}