Hi Benjamin,

If you can't switch the catcode of & before looking for the

optional argument, as David suggests, you can use the !

argument modifier to disallow a space token before the

first “[”:

\NewDocumentEnvironment { sdapsarray } { !o }

in which case:

\begin{sdapsarray}
  & b
\end{sdapsarray}

will work as expected, but:

\begin{sdapsarray} [] % note the space before the []
  & b
\end{sdapsarray}

and: