Extract a substring from a Chinese string variable

Question

Looks like the stringstrings package can do this:

\substring{This is a string.}{1}{4}

gives This, but it seems to be failed for non-ASCII characters:

\substring{这是一个句子。}{1}{4}

cannot give 这是一个.

Is there a similar package working for Chinese characters？

Answer 1

If you can switch to LuaLaTeX, it's straightforward to create a "wrapper" LaTeX macro that invokes the Lua function unicode.utf8.sub. (unicode.utf8.sub is a utf8-aware version of the standard string.sub function.)

\documentclass{article}
\usepackage{fontspec}
\setmainfont{MingLiU} % or whatever font you prefer
\newcommand\substring[3]{\directlua{%
   tex.sprint( unicode.utf8.sub ( "#1", #2 , #3 ) ) }}
\begin{document}
这是一个句子。

\substring{这是一个句子。}{1}{4}
\end{document}