Including a length-dependent page range using pdfpages [2nd Answer broken]

Question

Using the patch defining prelast for pdfpages throws an error. [Might it have to do with updates in pdfpages?]

I am using the second answer from Including a length-dependent page range using pdfpages by @Werner and edited by @DavidCarlisle.

Using the command \includepdf[pages=1-3]{dummy-l.pdf} everything works fine. But when I try \includepdf[pages=1-prelast]{dummy-l.pdf} instead, I receive the error:

Package pdfpages Error: Cannot find file `1-prelast'.

The goal is to read in pdfs of different lengths and color the last page of a text. When I removed \usepackage{xcolor} and all the \pagecolor{} commands, I still got the same error. However, since the color part is critical to my project I have left it in the code just in case.

Note that once this error has been fixed, I will have other questions from this project, but I didn't want to add them here.

\documentclass{amsart}
\usepackage[usenames,dvipsnames,svgnames]{xcolor}
    \pagecolor{white}
\usepackage{pdfpages}% http://ctan.org/pkg/pdfpages
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% DEFINE “prelast” to contain the pagenumber of the next-to-last page
% in pdfpages — from answer by Werner and David Carlisle found at
% https://tex.stackexchange.com/questions/62911/including-a-length-dependent-page-range-using-pdfpages
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\makeatletter
\def\AM@checklast#1-\END{%
  \edef\AM@tempii{\AM@trim@space{#1}}%
  \expandafter\ifx\expandafter\\\AM@tempii\\
    \@tempcntb=\AM@pagecount\relax
    \ifAM@DVIoutput
      \ifnum\AM@pagecount=1
        \@tempcntb\@tempcnta
      \fi
    \fi
    \advance\@tempcntb\@ne
    \whiledo{\@tempcnta<\@tempcntb}{%
      \AM@append{\the\@tempcnta}%
      \advance\@tempcnta\@ne
    }%
  \else
    \def\AM@cmp{last}%
    \ifx\AM@cmp\AM@tempii
      \let\AM@tempii\AM@pagecount
    \else\def\AM@cmp{prelast}% <------------------------- ADDED
      \ifx\AM@cmp\AM@tempii%                                  |
        \@tempcntb=\numexpr\number\AM@pagecount-1\relax%      |
        \edef\AM@tempii{\the\@tempcntb}%                      |
      \fi% <---------------------------------------------------
    \fi
    \AM@checkinteger{\AM@tempii}%
    \ifAM@integer
      \@tempcntb=\AM@tempii\relax
      \AM@checkpagenumber{\the\@tempcntb}%
      \ifnum\@tempcnta<\@tempcntb \def\AM@rel{<}\def\AM@inc{\@ne}%
      \else \def\AM@rel{>}\def\AM@inc{\m@ne}\fi
      \advance\@tempcntb\AM@inc
      \whiledo{\@tempcnta\AM@rel\@tempcntb}{%
        \AM@append{\the\@tempcnta}%
        \advance\@tempcnta\AM@inc
      }%
    \else
      \AM@setnewdocname{\AM@temp}%
    \fi
  \fi
}
\makeatother
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\begin{document}
\pagecolor{cyan!50!white}
Some Text
\clearpage % before changing color must be on next page
\pagecolor{white}
\includepdf[pages=1-prelast]{dummy-l.pdf} % <----- error
                             % can't find file `1-prelast'.
%\includepdf[pages=1-3]{dummy-l.pdf}       <----- works
\pagecolor{pink}
\includepdf[pages=last]{dummy-l.pdf}   %   <----- works
%\includepdf[pages=4]{dummy-l.pdf}         <----- works
\pagecolor{white}
Some more text.
\end{document}

Answer 1

I wouldn't patch pdfpages like this (and certainly not with code from a 10 years old answer). I would retrieve the page number as in the other answer, or in a more modern version.

Side remark: the usenames option is obsolete and will be removed in some future version.

\documentclass{amsart}
\usepackage{l3graphics}
\usepackage[dvipsnames,svgnames]{xcolor}
    \pagecolor{white}
\usepackage{pdfpages}% http://ctan.org/pkg/pdfpages
\ExplSyntaxOn
\newcommand\prelast{}
\newcommand\getprelast[1]
  {
   \graphics_get_pagecount:nN{#1}\l_tmpa_tl 
   \tl_set:Nx\prelast{\int_eval:n{\l_tmpa_tl-1}}
  }
\ExplSyntaxOff
\begin{document}
\pagecolor{cyan!50!white}
Some Text
\clearpage % before changing color must be on next page
\pagecolor{white}
\getprelast{example-image-a4-numbered.pdf} 
\includepdf[pages=1-\prelast]{example-image-a4-numbered} 
\pagecolor{pink}
\includepdf[pages=last]{example-image-a4-numbered}

\pagecolor{white}
Some more text.
\end{document}