Printing only the text that appears between two symbols

Question

I have a macro which contains some text:

\textmacro{This is some text.}

Normally, when compiled, it should just print text, like this:

This is some text.

Sometimes, the text contains “①”and “②”, such as this:

\textmacro{①  This is some text. ② This is some other text.}

When that happens, I need it to only print the text appearing between “①”and “②”, like this:

This is some text.

How can I make the macro print the text between “①” and “②” in #1, if those symbols appear, otherwise, to just print all of the text?

Answer 1

You want a ConTeXt answer, right? Whenever I catch myself thinking "I know how to do this in an ordinary programming language", I shunt the text I want to operate on into LuaTeX. Taking the template from the wiki:

% First, the Lua function that will process the string
\startluacode
    -- remember, using the userdata namespace prevents conflicts
    userdata = userdata or {}

    function userdata.printBetweenMarks(str, m1, m2)
        -- default delimiters
        m1 = m1 or '①'
        m2 = m2 or '②'

        match_pattern = string.format('%s(.*)%s', m1, m2)
        str = string.match(str, match_pattern) or str
        context(str)
    end
\stopluacode

% Secondly, the ConTeXt command that passes the string to the Lua function.

\def\textmacro#1%
    {\ctxlua{userdata.printBetweenMarks([===[#1]===])}}
    % Note that Lua sees the text as-is, so we have to wrap it in string
    % delimiters ourselves.
    % [=*[ is Lua's double-bracket string delimiter style; this way, the
    % code only fails if the text contains ']===]'.

EDIT: removed a print() statement I had used for debugging

Answer 2

\documentclass{article}
\makeatletter
\def\textmacro#1{\expandafter\@textmacro#1①②\@nil}
\def\@textmacro#1①#2②#3\@nil{%
  \ifx\relax#2\relax#1\else#2\fi}
\makeatother
\begin{document}
\textmacro{This is some text.}\par
\textmacro{①  This is some text. ② This is some other text.}\par
\textmacro{This is some text ①  This is some other text. ② This is some text.}
\end{document}