测试一个角色，无论其是否活跃（futurelet token）

Question

我想测试一个角色，而不考虑其当前的 catcode（即是否处于活动状态）。这与如何测试一个字符，而不考虑 catcode？，但是我不能使用，\pdf@strcmp因为我可能有“半”宏（例如，\emph还没有参数）。

\documentclass{minimal}
\usepackage[czech]{babel}% makes - active
\begin{document}
  \def\aaa{%
    \if a\noexpand\lookahead%
      (the letter a)%
    \else\if -\noexpand\lookahead%
      (a minus)%
    \else%
      (special treatment)
    \fi\fi%
  }
  \futurelet\lookahead\aaa -
  \futurelet\lookahead\aaa a
  \futurelet\lookahead\aaa \emph{c}
\end{document}

对我来说，更改前后的 catcode 并不是真正的解决方案，因为仅通过未来观察事物就会丢失原来的含义。

Answer 1

\ifx可用于直接比较标记。以下示例将标记存储在宏\aaaHyphenActive和中\aaaHyphenOther。在比较中，连字符标记通过解包\expandafter。

\documentclass{minimal}
\usepackage[czech]{babel}% makes - active

\begingroup
  \catcode`\-=\active
  \gdef\aaaHyphenActive{-}%
  \catcode`\-=12 %
  \gdef\aaaHyphenOther{-}%
\endgroup

\begin{document}
  \def\aaa{%
    \if a\noexpand\lookahead
      (the letter a)%
    \else\expandafter\ifx\aaaHyphenActive\lookahead
      (an active minus)%
    \else\expandafter\ifx\aaaHyphenOther\lookahead
      (a minus)%
    \else
      (special treatment)
    \fi\fi\fi
  }
  \futurelet\lookahead\aaa -
  \futurelet\lookahead\aaa a
  \futurelet\lookahead\aaa \emph{c}
\end{document}

Answer 2

您可以使用expl3。这是一个粗略的例子。

\documentclass{article}
\usepackage[czech]{babel}
\usepackage{xparse}
\ExplSyntaxOn
\NewDocumentCommand{\dosomething}{m}
 {
  \martin_do_something:n {#1}
 }
\cs_new_protected:Npn \martin_do_something:n #1
 {
  \peek_charcode:NTF a % test the first token against "a"
   {
    (the~letter~a)
   }
   {
    \peek_charcode:NTF - % test the first token against "-" (charcode only)
     {
      (a~hyphen)
     }
     {
      (special~treatment)
     }
   }
  #1
}
\ExplSyntaxOff

\begin{document}

\dosomething{abc}

\dosomething{-abc}

\dosomething{\emph{abc}}

\the\catcode`- % check that the hyphen was active

\catcode`-=12  % make it non active and try again

\dosomething{-abc}

\end{document}