Passing an argument in a different way in a self-made environment

Question

I want to pass an argument to a self-made tabular environment in the following format:

\begin{customTabular}{|c|c|}{'@'}

...such that the second argument (here '@', WITHIN PARANTHESIS) is treated as a column separator symbol by the LaTeX document. How this can be done? Please suggest the changes/modifications in the code attached.

The code for customTabular environment is as follows (here, mytabular is similar to customTabular):

\usepackage{xparse}

\ExplSyntaxOn
\NewDocumentEnvironment {mytabular} { O{\,} }
  {
    \char_set_catcode_other:N \&
    \char_set_catcode_alignment:n {`#1}
    \begin{tabular}
  }
  {
    \end{tabular}
  }
\ExplSyntaxOff

Answer 1

You need to make the argument an optional group (or perhaps mandatory), and to arrange to strip any quote marks before using as a number:

\documentclass{article}
\usepackage{xparse}
\ExplSyntaxOn
\NewDocumentEnvironment {mytabular} { O{c} m G{','} }
  {
    \char_set_catcode_other:N &
    \char_set_catcode_alignment:n { \__mytab_test:n {#3} }
    \begin{tabular}[#1]{#2}
  }
  {
    \end{tabular}
  }
\cs_new:Npn \__mytab_test:n #1 { \__mytab_test:w #1 ' #1 '' \q_stop }
\cs_new:Npn \__mytab_test:w #1 ' #2 ' #3 \q_stop { `#2 }
\ExplSyntaxOff

\begin{document}
\begin{mytabular}{cc}{'@'}
  a @ b\\
  c @ d
\end{mytabular}

\begin{mytabular}{cc}{@}
  a @ b\\
  c @ d
\end{mytabular}
\end{document}

The trick in removing ' tokens is that TeX is greedy in a match. So with

#1 ' #2 ' #3

any input containing a leading ' will have an empty #1, the 'argument' in #2 and rubbish left in #3. If there was no leading ', then #1 will be the same as #2 due to the set up macro, which supplies the input both with and without additional ' tokens.

Answer 2

If you don't require the quotes, your code works, provided you use square brackets for the argument. It's better style to also add the mandatory argument for the columns.

\documentclass{article}
\usepackage{xparse}

\ExplSyntaxOn
\NewDocumentEnvironment {mytabular} { O{\,} m }
  {
    \char_set_catcode_other:N \&
    \char_set_catcode_alignment:n {`#1}
    \begin{tabular}{#2}
  }
  {
    \end{tabular}
  }
\ExplSyntaxOff

\begin{document}

\begin{mytabular}[@]{cc}
  a @ b\\
  c @ d
\end{mytabular}

\begin{mytabular}{cc}
  a , b\\
  c , d
\end{mytabular}

\end{document}

If you want a syntax such as \begin{mytabular}{cc}[@], just exchange the arguments

\NewDocumentEnvironment {mytabular} { m O{\,} }
  {
    \char_set_catcode_other:N \&
    \char_set_catcode_alignment:n {`#2}
    \begin{tabular}{#1}
  }
  {
    \end{tabular}
  }