如何给题库中的题目打标签？

刚造完一个,,,还是用楼上的吧

\documentclass{article}

\ExplSyntaxOn
  \clist_new:N \g_tags_clist
  \clist_new:N \g_contents_clist

  \cs_new:Npn \__tagit:n #1 {
    \clist_gput_right:Nn \g_tags_clist { { #1 } }
  }

  \NewDocumentCommand{\tagit}{}{\__tagit:n}

  \newcounter{problem}

  \NewDocumentEnvironment{problem}{+b}{
    \stepcounter{problem}
    \clist_gput_right:Nn \g_contents_clist {\exp_not:N\begin{problem} #1 \exp_not:N\end{problem} }
    #1
  }{}

  \iow_new:N \g_tags_file 

  \cs_new:Npn \__print_by_tag:n #1 {
    \tl_clear:N \l_tmpa_tl
    \int_step_inline:nn { \the\value{problem} } {
      \exp_args:Ne \clist_if_in:nnT
        { \clist_item:Nn \g_tags_clist { ##1 } }
        { #1 }
        {
          \tl_put_right:Ne \l_tmpa_tl { \clist_item:Nn \g_contents_clist { ##1 } }
        }
    }
    \iow_open:Nn \g_tags_file { \jobname-#1.tex }
    \iow_now:Ne \g_tags_file { \l_tmpa_tl }
    \iow_close:N \g_tags_file
  }
  
  \NewDocumentCommand{\printByTag}{}{\__print_by_tag:n}
\ExplSyntaxOff


\begin{document}
\begin{problem}
aa
\tagit{tag1, tag2, tag3}
\end{problem}
\begin{problem}
aa
\tagit{tag2, tag3}
\end{problem}
\begin{problem}
aa
\tagit{tag1, tag2, tag3}
\end{problem}

\printByTag{tag1}
\end{document}

本着「不要重复造轮子」的原则，一个比较适合的包是texdox xsim

一个简单的用例在这里

给一个更完整点的例子（注意可能要手动删除.xsim文件以防止错误的结果）：

\documentclass{article}
\usepackage{xsim}
\usepackage{lipsum}
\DeclareExerciseCollection[label=A]{A}
\DeclareExerciseCollection[label=B]{B}
\DeclareExerciseCollection[label=C]{C}
\DeclareExerciseCollection[label=D]{D}
\DeclareExerciseTagging{label}
\xsimsetup{collect}
\begin{document}
\begin{exercise}[label=C]
  \textbf{C} \lipsum[2]
\end{exercise}
\begin{exercise}[label={A,B}]
  \textbf{A and B} \lipsum[2]
\end{exercise}
\begin{exercise}[label={B,C,D}]
    \textbf{B and C and D} \lipsum[2]
\end{exercise}
\begin{exercise}[label={D,A}]
    \textbf{D and A} \lipsum[2]
\end{exercise}

\section{AAA}
\printcollection{A}

\section{CCC}
\printcollection{C}

\section{DDD}
\printcollection{D}
\end{document}