Compiling the latexdiff when adding a subscript before a pmatrix environment causes compilation error

Question

Consider the following two simple .tex files:

test_old.tex:

\documentclass{article}
\usepackage{amsmath}

\begin{document}

$a \begin{pmatrix}a & b\end{pmatrix}$

\end{document}

and test_new.tex:

\documentclass{article}
\usepackage{amsmath}

\begin{document}

$a_2 \begin{pmatrix}a & b\end{pmatrix}$

\end{document}

Using latexdiff to highlight the differences between these two documents (using latexdiff 1.2.0 on windows 10) generates the following diff.tex file:

\documentclass{article}
%DIF LATEXDIFF DIFFERENCE FILE
%DIF DEL .\test_old.tex   Tue Mar 21 15:42:30 2017
%DIF ADD .\test_new.tex   Tue Mar 21 15:42:31 2017
\usepackage{amsmath}
%DIF PREAMBLE EXTENSION ADDED BY LATEXDIFF
%DIF UNDERLINE PREAMBLE %DIF PREAMBLE
\RequirePackage[normalem]{ulem} %DIF PREAMBLE
\RequirePackage{color}\definecolor{RED}{rgb}{1,0,0}\definecolor{BLUE}{rgb}{0,0,1} %DIF PREAMBLE
\providecommand{\DIFadd}[1]{{\protect\color{blue}\uwave{#1}}} %DIF PREAMBLE
\providecommand{\DIFdel}[1]{{\protect\color{red}\sout{#1}}}                      %DIF PREAMBLE
%DIF SAFE PREAMBLE %DIF PREAMBLE
\providecommand{\DIFaddbegin}{} %DIF PREAMBLE
\providecommand{\DIFaddend}{} %DIF PREAMBLE
\providecommand{\DIFdelbegin}{} %DIF PREAMBLE
\providecommand{\DIFdelend}{} %DIF PREAMBLE
%DIF FLOATSAFE PREAMBLE %DIF PREAMBLE
\providecommand{\DIFaddFL}[1]{\DIFadd{#1}} %DIF PREAMBLE
\providecommand{\DIFdelFL}[1]{\DIFdel{#1}} %DIF PREAMBLE
\providecommand{\DIFaddbeginFL}{} %DIF PREAMBLE
\providecommand{\DIFaddendFL}{} %DIF PREAMBLE
\providecommand{\DIFdelbeginFL}{} %DIF PREAMBLE
\providecommand{\DIFdelendFL}{} %DIF PREAMBLE
%DIF END PREAMBLE EXTENSION ADDED BY LATEXDIFF

\begin{document}

%DIF <  $\mathcal C\begin{pmatrix}a & b\end{pmatrix}$
\DIFdelbegin \DIFdel{$a \begin{pmatrix}a & b\end{pmatrix}$
}\DIFdelend %DIF >  $\mathcal C_2 \begin{pmatrix}a & b\end{pmatrix}$
\DIFaddbegin \DIFadd{$a_2 \begin{pmatrix}a & b\end{pmatrix}$
}\DIFaddend 

\end{document}

Compiling this file results in the following errors:

...\diff.tex:30: Forbidden control sequence found while scanning use of \UL@word. [}]
...\diff.tex:30: Paragraph ended before \UL@word was complete. [}]
...\diff.tex:34: Missing } inserted. [\end{document}]

What causes this? And how can I prevent it?

Answer 1

The reason this fails is because the ulem underlining commands break, if the inline-math expression has an array environment. This can be made to work by encapsulating the math expression using \mbox{$...$} (this would have to be done either in both old and new file, or in the resulting diff). As this is a known weakness of ulem latexdiff contained code to add the \mbox to its output where needed.

However, there was a pattern matching bug in latexdiff preventing this from working properly. At the time of writing this bug is fixed in the development version on github but not yet included in a release (it will be included from 1.2.1 onwards). If you do not want to clone the github version, you can apply these two patches by hand:

    $begin2=pos($delblock) - $len2;
    $mathblock="%\n\\mbox{$AUXCMD\n$1\n}$AUXCMD\n";
-   next unless $mathblock =~ m/\{$ARRENV\}/ ;
+   next unless ( $mathblock =~ /ARRAYBLOCK/ or $mathblock =~m/\{$ARRENV\};
    substr($delblock,$begin2,$len2)=$mathblock;
    pos($delblock) = $begin2 + length($mathblock);

and

@@ -2608,7 +2615,7 @@ sub postprocess {
    $len2=length($&);
    $begin2=pos($addblock) - $len2;
    $mathblock="%\n\\mbox{$AUXCMD\n$1\n}$AUXCMD\n";
-   next unless $mathblock =~ m/\{$ARRENV\}/ ;
+   next unless ( $mathblock =~ /ARRAYBLOCK/ or $mathblock =~ m/\{$ARRENV\}/) ;
    substr($addblock,$begin2,$len2)=$mathblock;
    pos($addblock) = $begin2 + length($mathblock);
   }