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Message-ID:  <CANfjC09icf4nCppps_wcg6EOb_-VmcWFX2JMNWOaLFWCRoz+gA@mail.gmail.com>
Date:         Sun, 31 Mar 2019 18:57:32 -0300
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              <LATEX-L@LISTSERV.UNI-HEIDELBERG.DE>
Sender:       Mailing list for the LaTeX3 project <LATEX-L@LISTSERV.UNI-HEIDELBERG.DE>
From:         Phelype Oleinik <phe.h.o1@GMAIL.COM>
Subject: Re: Rescanning first token of environment when no optional argument is given
To:           LATEX-L@LISTSERV.UNI-HEIDELBERG.DE
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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 =E2=80=9C[=E2=80=9D:

\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:

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

won't see the optional argument anymore.

Best,

Phelype

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<div dir=3D"ltr"><div dir=3D"ltr"><div dir=3D"ltr"><div dir=3D"ltr"><div>Hi=
 Benjamin,<br></div><div><br></div><div>If you can&#39;t switch the catcode=
 of &amp; before looking for the</div><div>optional argument, as David sugg=
ests, you can use the !</div><div>argument modifier to disallow a space tok=
en before the</div><div>first =E2=80=9C[=E2=80=9D:</div><div><br></div><div=
>\NewDocumentEnvironment { sdapsarray } { !o }</div><div><br></div><div>in =
which case:<br><br>\begin{sdapsarray}<br>=C2=A0 &amp; b<br>\end{sdapsarray}=
</div><div><br></div><div>will work as expected, but:</div><div><br>\begin{=
sdapsarray} [] % note the space before the []<br>=C2=A0 &amp; b<br>\end{sda=
psarray}</div><div><br></div><div>and:</div><div><div><br>\begin{sdapsarray=
}</div><div>=C2=A0 []<br>=C2=A0 &amp; b<br>\end{sdapsarray}</div><div><br><=
/div><div>won&#39;t see the optional argument anymore.</div><div><br></div>=
<div>Best,</div><div><br></div><div>Phelype<br></div></div></div></div></di=
v></div>

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