Compile error with redefined bmatrix and align environment

Question

I just spent a couple of hours finding the reason for compiling errors in the last example in here:

\documentclass[a4paper,draft=true, headsepline=on, twoside
, 11pt,
]{scrbook}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}

\usepackage{amsmath}


%matrix environment redef
\makeatletter
\renewcommand*\env@matrix[1][*\c@MaxMatrixCols c]{%
  \hskip -\arraycolsep
  \let\@ifnextchar\new@ifnextchar
  \array{#1}}
\makeatother






\begin{document}

\begin{equation*}
  \begin{bmatrix}
     & b 
  \end{bmatrix}
\end{equation*}

\begin{align*}
  \begin{bmatrix}
   ~  & b 
  \end{bmatrix}
\end{align*}

\begin{align*}
  \begin{array}{cc}
     & b 
  \end{array}
\end{align*}  


\begin{align*}
  \begin{bmatrix}
     & b 
  \end{bmatrix}
\end{align*}  


\end{document}

The error does not appear if I do not redefine bmatrix environment like discussed here: How does this macro for augmented matrices work?

However, the errors only seam to appear in this special case of using the align environment with a matrix that has empty entries in first column. Do you understand where these errors come from? I have no idea, but this could certainly help later on if similar problems occur.

Answer 1

The scanning for the optional argument confuses the alignment parser, due to the empty first cell in the matrix.

You're luckier with xparse:

\documentclass[a4paper,draft=true, headsepline=on, twoside
, 11pt,
]{scrbook}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}

\usepackage{amsmath,xparse}

%matrix environment redef
\makeatletter
\RenewDocumentCommand\env@matrix{O{*\c@MaxMatrixCols c}}{%
  \hskip -\arraycolsep
  \let\@ifnextchar\new@ifnextchar
  \array{#1}}
\makeatother

\begin{document}

\begin{align*}
  \begin{bmatrix}
     & b 
  \end{bmatrix}
\end{align*}

\end{document}

Answer 2

You can add an extra level of bracing to make the lookahead safe.

\documentclass[a4paper,draft=true, headsepline=on, twoside
, 11pt,
]{scrbook}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}

\usepackage{amsmath}


%matrix environment redef
\makeatletter
\renewcommand*\env@matrix[1][*\c@MaxMatrixCols c]{%
  \hskip -\arraycolsep
  \let\@ifnextchar\new@ifnextchar
  \array{#1}}

\renewenvironment{bmatrix}
{{\ifnum`}=0 \fi\left[\env@matrix}
{\endmatrix\right]\ifnum`{=0 \fi}}
\makeatother






\begin{document}

\begin{equation*}
  \begin{bmatrix}
     & b 
  \end{bmatrix}
\end{equation*}

\begin{align*}
  \begin{bmatrix}
   ~  & b 
  \end{bmatrix}
\end{align*}

\begin{align*}
  \begin{array}{cc}
     & b 
  \end{array}
\end{align*}  


\begin{align*}
  \begin{bmatrix}
     & b 
  \end{bmatrix}
\end{align*}  


\end{document}