how to automatically convert \cases to \begin{cases} \end{cases}?

Question

Is there some smart macro I could add in the preamble to handle this LaTeX auto-generated code?

I use Maple's latex() command to convert results to LaTeX that I then compile using texlive in a separate document.

Sometimes it generates the wrong LaTeX or broken LaTeX. Here is an example of one such case:

The above output is send to a latex file and wrapped inside \begin{equation} and \end{equation}. I picked equation environment to put the result inside, but I can change this if it helps in conversions.

This is all done using an automated script. I do not use any of Maple style sheets or anything else. I simply just take the latex() output and process that using texlive standard setup.

I have hundreds of such equations, so manually editing any broken Maple LaTeX output is not possible and since each time I re-run the script the edits will be lost.

Is there a way to make a macro that will convert the above to use \begin{cases} with an enclosing \end{cases} using some smart replacement rule to make LaTeX and amsmath happy?

Here is the error, and a MWE:

MWE

\documentclass[11pt,notitlepage]{article}
\usepackage{amsmath}
\usepackage{amsfonts}
\usepackage{amssymb}
%smart renewcommand here? or \let?
\begin{document}

\begin{equation}
x-
\cases{-2\,\sqrt {-y \left( x \right) }&$y \left( x \right) \leq 0$\cr 
2\,\sqrt {y \left( x \right) }&$0<y \left( x \right) $\cr}+{\it \_C1}=0_{{1}}
\end{equation}

\end{document} 

and

pdflatex foo.tex

(/usr/local/texlive/2014/texmf-dist/tex/latex/amsfonts/umsa.fd)
(/usr/local/texlive/2014/texmf-dist/tex/latex/amsfonts/umsb.fd)

! Package amsmath Error: Old form `\cases' should be \begin{cases}.

See the amsmath package documentation for explanation.
Type  H <return>  for immediate help.
 ...                                              

l.9 \cases
          {-2\,\sqrt {-y \left( x \right) }&$y \left( x \right) \leq 0$\cr
? 

Answer 1

The old \cases{...} form is an unabridged copy of the same macro in Plain TeX and is in the LaTeX kernel just because it was in Plain.

The environment form was introduced in amsmath.sty issued, if I remember correctly, in 1994, so I'm quite surprised that Maple insists in using deprecated commands and doesn't exploit amsmath.

What you want can be done, though. Possibly not as robust as one can hope, but it's a hack, after all.

The key is using mathtools in order to overcome the deficiencies of the old \cases command.

\documentclass[11pt,notitlepage]{article}

\usepackage{amsmath,mathtools}
\usepackage{amssymb}

\let\amscases\cases
\makeatletter
\def\cases{\@ifnextchar\bgroup\plaincases\amscases}
\def\plaincases#1{\begin{cases*}#1\end{cases*}}
\makeatother



\begin{document}

\begin{equation}
x-
\cases{-2\,\sqrt {-y \left( x \right) }&$y \left( x \right) \leq 0$\cr 
2\,\sqrt {y \left( x \right) }&$0<y \left( x \right) $\cr}+{\it \_C1}=0_{{1}}
\end{equation}

\begin{equation}
x-
\begin{cases}
  -2\,\sqrt {-y \left( x \right) }&y \left( x \right) \leq 0\\
  2\,\sqrt {y \left( x \right) }&0<y \left( x \right) 
\end{cases}+{\it \_C1}=0_{{1}}
\end{equation}

\begin{equation}
x-
\begin{cases}
  -2\sqrt{-y(x)} & y(x) \leq 0\\
  2\sqrt{y(x)}   & 0 < y (x)
\end{cases}+\mathit{\_C1}=0_{1}
\end{equation}

\end{document} 

The second example shows that the environment form is typeset correctly, the third is the “really correct” LaTeX input, without useless \left and \right and the obsolete and deprecated \it command.

Answer 2

It should be easy to replace all occurences of \cases by, say, \Cases. Then, with some additional modifications:

\documentclass[11pt,notitlepage]{article}
\usepackage{amsmath}
\usepackage{amsfonts}
\usepackage{amssymb}
%smart renewcommand here? or \let?
\def\Cases#1{\begin{cases}#1\end{cases}}
\begin{document}

\begin{equation}
x-
\Cases{-2\,\sqrt {-y \left( x \right) }&y \left( x \right) \leq 0\cr 
2\,\sqrt {y \left( x \right) }&0<y \left( x \right) \cr}+{\it \_C1}=0_{{1}}
\end{equation}

\end{document}