How do I create a specially delimited command in `xparse`

Question

Using \def I can define a command as follows:

\def\aecmd#1 located at (#2);{---definition---}

I can recreate this with xparse as follows:

\NewDocumentCommand\aecmd{ u{~located~at~(}u{);}}{----definition---}

However, I have a situation where I want to be able to write a command with an (almost final) optional argument. This would be a pain in the neck to do with \def.

I'd like a command which could be expressed as either

\aecmd{Manditory content}[optional content] located at (location);

or

\aecmd{Manditory content} located at (location);

Essentially, I would like something that works like r but instead of taking a single token as the delimiters can take whole strings. Let's call this hypothetical argument specifier b

Then my desired command could be defined along these lines:

\NewDocumentCommand{ mob{~located~at~(}{);}{ ---something--- }

I'm aware I could write something like that following:

\NewDocumentCommand\aecmd{ mou{~located~at~(}u{);}{---definition---}

Further, I suppose that I could test whether or not something was passed in the 3rd argument and raise an error if there was anything there.
But this just seems clunky. It doesn't feel right to me.

Any suggestions about how to do this better?

Answer 1

xparse? Pshaw!

EDITED to capture syntax error if optional argument is unbracketed.

\documentclass{article}
\usepackage{parskip}

\newcommand\aecmd[1]{\def\mandatory{#1}\aecmdaux}
\def\aecmdaux#1 located at (#2);{\def\location{#2}\aecmdauxA{#1}}
\newcommand\aecmdauxA[1]{\setbox0=\hbox{%
  \if[#1[\gdef\next{\aecmdauxB#1}\else \gdef\next{syntax error}\fi}%
  \next%
}
\newcommand\aecmdauxB[1][]{%
  Optional: #1\\ Mandatory: \mandatory\\ location: \location}

\begin{document}
\aecmd{Colonel Mustard} located at (the library);

\aecmd{Colonel Mustard}[with a wrench] located at (the library);

\aecmd{Colonel Mustard} on 4th street located at (the library);
\end{document}

If one does not like the output of "syntax error" then in the definition of \aecmdauxA, the words "syntax error" can be replaced with \aecmdauxB and the effect will be to ignore the stray words in the syntax, while still producing the mandatory and location arguments.

Answer 2

With xparse, but I find such syntax a very bad idea, notwithstanding TikZ using it.

\documentclass{article}
\usepackage{xparse}

\ExplSyntaxOn
\NewDocumentCommand{\aecmd}{mou{located~at~(}u{);}}
 {
  \tl_if_blank:nF { #3 } { \ERROR }
  \noindent
  #1~(mandatory)\\
  \IfValueT{#2}{#2~(optional)\\}
  #4~(final)
 }
\ExplSyntaxOff

\begin{document}

\aecmd{Mandatory content}[optional content] located at (location);

\aecmd{Mandatory content} located at (location);

\aecmd{Colonel Mustard} on 4th street located at (the library);

\end{document}

Note that the third call raises an error.