special charactor handling in xstring

Question

I want to handle a string of characters to get my desired string by the package xstring, here is my code:

    \documentclass[12pt,a4paper]{article}
    \usepackage{xstring}

    \begin{document}

    \StrBefore{aaa \bfseries bbb;some other text}{;}[\temp]
\temp

    \end{document}

The output string I want is "aaa \bfseries bbb",but the code fails to compile. I think the reason is the existence of "\bfseries", the character "\" causes the failure. Anyone can help me with this problem? Thank you.

Answer 1

I hope I understand what you want (printing \bfseries as a result and not interpreting it). This can be done by changing it to a string with \string or \detokenize. Note that in the default font the backslash is printed as high ticks if you use this option.

\documentclass[12pt,a4paper]{article}
\usepackage{xstring}

\begin{document}

\StrBefore{aaa \detokenize{\bfseries} bbb;some other text}{;}[\temp]
\temp\\
\StrBefore{\detokenize{aaa \bfseries bbb;some other text}}{;}[\temp] % works, too
\texttt{\temp}% with \texttt the backslash is printed

\end{document}

Answer 2

You need to prevent the expansion of \bfseries:

\documentclass[12pt,a4paper]{article}
 \usepackage{xstring}

 \begin{document}
 \noexpandarg  
 \StrBefore{aaa  \bfseries bbb;some other text}{;}[\temp]
 \temp

 \end{document}

Answer 3

Here's a LuaLaTeX-based solution. (Lua has a very powerful string manipulation library.) The answer sets up a LaTeX macro called \StringBefore, which calls a Lua function that does the actual work. The macro \StringBefore is expandable; it can be made the argument of some other macro, as is done in the code below. Incidentally, I'm assuming that if the character(s) in the second argument aren't found in the search string, nothing should be returned.

% !TeX program = lualatex
\documentclass{article}
\usepackage{luacode} % for "\luastringN" macro and "luacode" env.
\begin{luacode} 

function string_before ( str, patt ) 
  n = string.find ( str, patt ) 
  if n then -- string.find made (at least) one match...
      tex.sprint ( string.sub ( str, 1, n-1) )
  else   -- no match -> print nothing
      tex.sprint ( "" ) 
  end
end

\end{luacode}
\newcommand\StringBefore[2]{\directlua{ 
    string_before(\luastringN{#1},\luastringN{#2})}}
\begin{document}

\StringBefore{aaa {\bfseries bbb} ccc;some other text}{;}

\newcommand{\temp}{\StringBefore{ddd {\bfseries eee} \emph{ggg} hhh;dummy text}{;}}

\temp

\StringBefore{aaa {\bfseries bbb} ccc;some other text}{u} % no match

zzz
\end{document}