Use parameters as delimiters in \NewDocumentCommand

Question

Using \NewDocumentCommand, it's possible to specify a delimited argument (e.g. r() for a required argument in parentheses). Is it possible to use other arguments in the same command as those delimiters? For example, is it possible to do something like this, or are there any workarounds?

\documentclass{article}

\NewDocumentCommand\getbetween { m m r#1#2 } { #3 }

\begin{document}

\getbetween() (abc)

\end{document}

(The code above produces the errors that there is an "Illegal parameter number in definition of \__tl_map_1:w." and that there is an "Invalid argument type '##' in command '\getbetween'.")

Answer 1

If you want this, you can do it in two steps:

\documentclass{article}

\NewDocumentCommand\getbetweenx{}{}

\NewDocumentCommand\getbetween { m m}{%
 \RenewDocumentCommand\getbetweenx{r#1#2 } {arg was: ``##1''}%
 \getbetweenx}

\begin{document}

\getbetween() (abc)

\end{document}