Regression in LaTeX parsing

[simonrob]

I've just upgraded from an older version of pandoc (1.19.2.4) to 2.1.1 and encountered an issue with LaTeX parsing. The older version handled this with no problems, but the current version gives the following error:

Error at "source" (line 2, column 26): unexpected _

Sample LaTeX file:

\documentclass{article}
\usepackage[sortlocale=en_GB]{biblatex}
\begin{document}
Test
\end{document}