Inexplicable error after importing syntax package

Question

I'm using the LaTeX-KOMA-template (based on KOMA-script as the name suggests) provided by Karl Voit for writing a thesis. I can compile the main.tex perfectly fine, until I import the syntax package. This happens even if I compile the template 'as is', without any other packages imported. I get the following errors I simply can't wrap my head around:

! Missing \endcsname inserted. \protect l.232 \input{template/declaration_TU_Graz} %% Statutory Declaration The control sequence marked should not appear between \csname and \endcsname.

! Extra \endcsname. \InputIfFileExists ...\csname #1-@alias\endcsname \relax \expandafter \@seco... l.232 \input{template/declaration_TU_Graz} %% Statutory Declaration I'm ignoring this, since I wasn't doing a \csname.

! TeX capacity exceeded, sorry [input stack size=5000]. \@inmathwarn ...latex@warning {Command \protect #1 invalid in math mode}\fi l.232 \input{template/declaration_TU_Graz} %% Statutory Declaration If you really absolutely need more capacity, you can ask a wizard to enlarge me.

How can this happen by simply importing the syntax package?

How to reproduce:

1. Download LaTeX-KOMA-template and extract it
2. Typeset main.tex -> It works as expected
3. Insert \usepackage{syntax} in designated MISC usepackages area of main.tex
4. Typeset main.tex again -> The error occurs.

System: texmaker 4.4.1 on MacTex-2015 (TeX Live 2015) without any package updates running on OS X Yosemite 10.10.3.

Please let me know if you need any additional information!

Answer 1

The Syntax package redefines the underscore character, so when you use it in filenames it is not read as it is but interpreted as a macro and expanded. Simple solution: avoid underscore in filenames.

Not so pretty solution: capture the original underscore char in a macro before the syntax package changes its meaning (at begin document) and then use the macro instead of the underscore:

\documentclass{article}
\usepackage{syntax}
\let\UnderScore_
\begin{document}
\input{bla\UnderScore bla}
\end{document}

A more general solution: revert the meaning of _ (maybe locally) using

\catcode`\_=11

This will make _ a normal character again. You can re-activate syntax's redefinition by doing

\catcode`\_=\active

You can put these two in two macros \makeunderscoreletter and \makeunderscoreactive and use it locally:

\documentclass{article}
\usepackage{syntax}

\newcommand{\makeunderscoreletter}{\catcode`\_=11}
\newcommand{\makeunderscoreactive}{\catcode`\_=\active}

\begin{document}
\makeunderscoreletter
\input{bla_bla} % interprets _ as letter
\makeunderscoreactive

my_variable % uses syntax redefinition

\end{document}

Answer 2

To expand on the accepted answer, the syntax package also has an optional argument, nounderscore, specifically to fix this problem. Loading the package using \usepackage[nounderscore]{syntax} fixes your problem.