How to remove any kind of spaces at the beginning of something

Question

This is another chapter of the story "how to use a moodle database for my purposes". The accepted answer to this question gives a very effective way to retrieve informations from an exercise database structured as follows

\begin{multi}{}
  Text
  \item first answer
  \item* correct anser
  ...
 \end{multi}

Now, the object retrieved as "nth answer" contains all kind of spacing, for instance

  \item
first answer

will store the \par.

I would like to erase all kind of "blank" spaces before the start of the actual aswer, for editing purpose.

---

An example for the Wipet answer which does not fit in a comment. If I set

  \documentclass{article}
  \usepackage{getitems}
  \begin{document}
\def\item{\futurelet\next \itemA}
  \def\itemA{\ifx\next\par \expandafter\itemB \fi}
  \long\def\itemB#1{\item}
\def\A{
A}
\def\B{%
B}
X\expandafter\item\A X
X\expandafter\item\B X
 \end{document}

The output is

 X
 AX
 XBX

So \expandafter\item\A does not work (it includes a linebreak before A) while \expandafter\item\B works (it does not include a linebreak before B)

Answer 1

The \item control sequence consumes all space after it at during tokenization. So, you want to remove only blank lines tokenized as \par. If you don't asking for expandable macro, the solution can look like this:

\def\item{\futurelet\next \itemA}
\def\itemA{\ifx\next\par \expandafter\itemB \fi}
\long\def\itemB#1{\item}
test:
aaa%
\item
bbb
\bye

Another approach: Suppose we have defined something like this:

\def\TEXT{
\item
a b c
\item
d e f
g h i
}

and we want to create a new macro \TEXT where all \par immediately followed by \item are removed. It means that the two \pars after first \item are removed in our example but the \par between d e f and g h i is kept. We can do this by

\rempars\TEXT

This macro changes the body of the \TEXT as mentioned above. This macro can be defined by:

\long\def\addto#1#2{\expandafter\def\expandafter#1\expandafter{#1#2}}
\long\def\afterfi#1#2\fi{\fi#1}
\def\rempars #1{%
   \let\orig=#1%
   \def\tmp{}%
   \expandafter\remparsA #1\item\par\endrempars
   \expandafter\def\expandafter#1\expandafter{\tmp}%
   \ifx#1\orig\else \afterfi{\rempars#1}\fi
}
\long\def\remparsA #1\item\par#2\endrempars{%
   \if\relax\detokenize{#2}\relax \addto\tmp{#1}%
   \else \addto\tmp{#1\item}\afterfi{\remparsA #2\endrempars}\fi
}

Answer 2

Add something to the code I suggested last time.

\documentclass{article}
\usepackage{amsthm}
\usepackage{enumitem}
\newcounter{exercise}
\ExplSyntaxOn
\NewDocumentEnvironment{multi}{m +b}
 {
  \par\addvspace{\topsep}\setlength{\parindent}{0pt}
  \stepcounter{exercise}
  \textbf{Exercise~\theexercise}~--~#1\par\nobreak
  \moodle_multi:n { #2 }
  \par\addvspace{\topsep}
 }
 {}
\NewDocumentCommand{\showcorrect}{}
 {
  \bool_gset_true:N \g_moodle_multi_correct_bool
 }
\NewDocumentCommand{\hidecorrect}{}
 {
  \bool_gset_falsee:N \g_moodle_multi_correct_bool
 }
\bool_new:N \g_moodle_multi_correct_bool
\seq_new:N \l__moodle_multi_text_seq
\tl_new:N \l__moodle_multi_text_tl
\tl_new:N \l__moodle_multi_moretext_tl
\tl_new:N \l__moodle_multi_answers_tl
\cs_new_protected:Nn \moodle_multi:n
 {
  \regex_split:nnN { {(enumerate|itemize)} } { #1 } \l__moodle_multi_text_seq
  \tl_set:Nx \l__moodle_multi_answers_tl { \seq_item:Nn \l__moodle_multi_text_seq { -1 } }
  \tl_set:Nn \l__moodle_multi_text_tl { #1 }
  \regex_replace_once:nnN { (.)\u{l__moodle_multi_answers_tl} } { \1 } \l__moodle_multi_text_tl
  \seq_set_split:NnV \l__moodle_multi_text_seq { \item } \l__moodle_multi_answers_tl
  % the first item will contain the question text
  \seq_pop_left:NN \l__moodle_multi_text_seq \l__moodle_multi_moretext_tl
  \tl_use:N \l__moodle_multi_text_tl
  \tl_use:N \l__moodle_multi_moretext_tl
  % now we rebuild the answers
  \begin{enumerate}[label=(\Alph),nosep,labelsep=0.75em]
  \use:e
   {% add the first \item and put it also between the other items
    __moodle_multi_item: \seq_use:Nn \l__moodle_multi_text_seq { __moodle_multi_item: }
   }
  \end{enumerate}
 }
\cs_new_protected:Nn __moodle_multi_item:
 {
  \item 
  \peek_charcode_remove:NT *
   {% there is an asterisk
    \bool_if:NT \g_moodle_multi_correct_bool { __moodle_multi_correct: }
    __moodle_multi_ignore:
   }
 }
\cs_new_protected:Nn __moodle_multi_correct:
 {
  \makebox[0pt][r]{\makebox[0pt][l]{$^{\scriptscriptstyle*}$}\hspace{\labelsep}}
 }
\cs_new_protected:Nn __moodle_multi_ignore:
 {% remove blank spaces and \par
  \peek_regex_remove_once:nTF { [\s\c{par}]* } {} {}
 }
\ExplSyntaxOff
\begin{document}
\begin{multi}{Name of the question}
  Text of the question
  \begin{itemize}
  \item a
  \item b
  \end{itemize}
  More text
  \begin{enumerate}
  \item a
  \item b
  \end{enumerate}
  More text
  \item first answer
  \item
second answer
  \item* the starred is the correct answer
\end{multi}
\showcorrect
\begin{multi}{Name of the question}
  Text of the question
  \item first answer
  \item second answer
  \item*
the starred is the correct answer
\end{multi}
\end{document}