xparse modal command – custom macro like \bfseries

Question

I would like to create a macro command that would put everything inside brackets for example or any other character, till the end of a line or till the end of a group {\inchar ...}. Something like a custom \bfseries command. Using default commands or xparse. It is interesting to see several implementation if possible.

That is, such an inputs should produce corresponding outputs

---

text text \inchar() text text text text

some other text on a new line

                      ↓

text text (text text text text)
some other text on a new line

---

text text \inchar|] text text text text\\
some other text on a new line

                      ↓

text text |text text text text]
some other text on a new line

---

text text \inchar\{\} text text text text\par
some other text on a new line

                      ↓

text text {text text text text}
some other text on a new line

---

Also it should work in amsmath environments

\begin{multline*}
abc\\
\inchar() def\\
ghi \inchar\{\} jkl + mno
\end{multline*}

                      ↓

abc
                                          (def)
                                                                           ghi {jkl + mno}

Answer 1

A first try: \autoScoped{<arg>} opens a scope which can be auto ended by \\ or \par, and then uses <arg>. Note this is not the usual way that LaTeX should be used and this first try is far from complete and might break many things.

Support for lists (including trivlist) is added. After trying, I find I am not capable of patching amsmath package yet. Sorry.

\documentclass{article}
\usepackage{multicol}
\usepackage{xpatch}

\makeatletter
\newif\if@inscope

% start a local scope, nesting is not supported
\def\autoScoped{\@inscopetrue\ifmmode[\else\bfseries\fi}
% end a local scope
\def\end@scope@if@in{%
  \if@inscope
    \ifmmode]\else\mdseries\fi
    \@inscopefalse
  \fi}

% patch variants of \\
\@tfor\@tempa:=%
  \@normalcr % normal \\
  \@centercr % inside trivlist
\do{%
  \expandafter\xpretocmd\@tempa
    {\end@scope@if@in}
    {}{\fail}%
}
\let\\\@normalcr

% patch \par
\def\par@delimited{\end@scope@if@in\@@par}
\let\par\par@delimited
\def\@par{\let\par\par@delimited\par}
\def\@setpar#1{%
  \def\par{\end@scope@if@in#1}\def\@par{\end@scope@if@in#1}}
\makeatother


\begin{document}

\newcommand\test[1][]{
  % case 1: delimited by \\
  #1\normalfont abc def \autoScoped ghi \\ jkl \par
  % case 2: delimited by explicit \par
  #1\normalfont abc def \autoScoped ghi \par jkl \par
  % case 3: delimited by implicit \par (consecutive newlines)
  #1\normalfont abc def \autoScoped ghi

  jkl \par
}

\setlength{\columnseprule}{.4pt}
\begin{multicols}{2}  
  \subsection*{Normal case}
  \test

  \subsection*{Inside trivlist and list}
  \begin{enumerate}
    \test[\item]
  \end{enumerate}

  \begin{center}
    \test
  \end{center}

  {\raggedleft \test \par}
\end{multicols}

\end{document}