Unsigned integer variable in a regex

Question

I need chapter/section specific labels to be generated dynamically for each chapter and section. I have modified the chapter and section command to do that. I use a regex based string manipulation and the following scheme to achieve it (MWE uses an article class to illustrate) -

\documentclass{article}
\usepackage{expl3,l3regex}
\ExplSyntaxOn
\NewDocumentCommand{\compactlabel}{m}
{
    \tl_set:Nn \l_tmpa_tl { #1 }
    \regex_replace_all:nnN {[^a-zA-Z]} {\ } \l_tmpa_tl
    \regex_replace_all:nnN {\ ([a-zA-Z]{1,2}\ )*} {\ } \l_tmpa_tl
    \regex_replace_all:nnN {^[a-zA-Z]{1,2}\ } {\ } \l_tmpa_tl
    \regex_replace_all:nnN {\ [a-zA-Z]{1,2}$} {\ } \l_tmpa_tl
    \regex_replace_all:nnN {^\ +} {} \l_tmpa_tl
    \regex_replace_all:nnN {\ +$} {} \l_tmpa_tl
    \regex_replace_all:nnN {\ +} {\ } \l_tmpa_tl
    \regex_replace_all:nnN {\ } {_} \l_tmpa_tl
    \regex_replace_all:nnN {(^\w{1,20})\w*} {\1} \l_tmpa_tl
    \regex_replace_all:nnN {_$} {} \l_tmpa_tl
    \tl_use:N \l_tmpa_tl
}
\ExplSyntaxOff
\begin{document}
% \sidebyside is a temporary command just to display and illustrate the point
\newcommand*{\sidebyside}[1]{
        #1

        \compactlabel{#1}
}

    \sidebyside{A 1st Mathematical Primer for the universe of chimpmunks}
\end{document}

My objectives :

1. The argument string must be cleared of all one and two letter words as well as all numbers and non-characters.
2. All multiple or single spaces the in the string must be replaced by an underscore.
3. The last or first character of the output must not be an underscore.
4. The string size should be curtailed at 20**.

So in the MWE, I see this as output -

A 1st Mathematical Primer for the universe of chimpmunks
mathematical_primer

** Notice the magic number 20 in the line \regex_replace_all:nnN {(^\w{1,20})\w*} {\1} \l_tmpa_tl above.

** Ideally I want this number to be a variable, default set at 20. So if I call \compactlabel{#1} it should mean 20 as above. But I would like to also call something like \compactlabel[10]{#1} and get the string size to be curtailed by 10.

PS: To be honest I do not claim to completely understand the expl3 or l3regex syntax. I copied and adapated it from tex.stackexchage forum.

Answer 1

Use an optional argument:

\documentclass{article}
\usepackage{xparse}
%\usepackage{l3regex} % only for releases earlier than June 2017

\ExplSyntaxOn
\NewDocumentCommand{\compactlabel}{O{20}m}
 {
  \tl_set:Nn \l_tmpa_tl { #2 }
  \regex_replace_all:nnN {[^a-zA-Z]} {\ } \l_tmpa_tl
  \regex_replace_all:nnN {\ ([a-zA-Z]{1,2}\ )*} {\ } \l_tmpa_tl
  \regex_replace_all:nnN {^[a-zA-Z]{1,2}\ } {\ } \l_tmpa_tl
  \regex_replace_all:nnN {\ [a-zA-Z]{1,2}$} {\ } \l_tmpa_tl
  \regex_replace_all:nnN {^\ +} {} \l_tmpa_tl
  \regex_replace_all:nnN {\ +$} {} \l_tmpa_tl
  \regex_replace_all:nnN {\ +} {\ } \l_tmpa_tl
  \regex_replace_all:nnN {\ } {_} \l_tmpa_tl
  \regex_replace_all:nnN {(^\w{1,#1})\w*} {\1} \l_tmpa_tl
  \regex_replace_once:nnN {_$} {} \l_tmpa_tl
  \tl_use:N \l_tmpa_tl
 }
\ExplSyntaxOff

\begin{document}

A 1st Mathematical Primer for the universe of chimpmunks

\texttt{\compactlabel{A 1st Mathematical Primer for the universe of chimpmunks}}

\texttt{\compactlabel[10]{A 1st Mathematical Primer for the universe of chimpmunks}}

\end{document}

Note that l3regex has been included in the kernel with the last release of expl3.

A revised version with comments; I added also lowercasing the final string. The last two examples end up with the same string, because of removal of a trailing underscore.

\documentclass{article}
\usepackage{xparse}
%\usepackage{l3regex} % only for releases earlier than June 2017

\ExplSyntaxOn
\NewDocumentCommand{\compactlabel}{O{20}m}
 {
  \tl_set:Nn \l_tmpa_tl { #2 }
  % remove non alphabetic/space characters
  \regex_replace_all:nnN {[^a-zA-Z\s]} {} \l_tmpa_tl
  % remove one or two character words
  \regex_replace_all:nnN {\b[a-zA-Z]{1,2}\b} {} \l_tmpa_tl
  % remove leading and trailing spaces
  \regex_replace_once:nnN {\A\s+} {} \l_tmpa_tl
  \regex_replace_once:nnN {\s+\Z} {\ } \l_tmpa_tl
  % change runs of spaces into one underscore
  \regex_replace_all:nnN {\s+} {_} \l_tmpa_tl
  % curtail at the stated limit (default 20)
  \regex_replace_all:nnN {(^\w{1,#1})\w*} {\1} \l_tmpa_tl
  % remove a possible trailing underscore
  \regex_replace_once:nnN {_\Z} {} \l_tmpa_tl
  % use the result
  \tl_set:Nx \l_tmpa_tl { \str_lower_case:f { \tl_use:N \l_tmpa_tl } }
  \tl_use:N \l_tmpa_tl
 }
\ExplSyntaxOff

\begin{document}

A 1st Mathematical Primer for the universe of chimpmunks

\texttt{\compactlabel{A 1st Mathematical Primer for the universe of chimpmunks}}

\texttt{\compactlabel[10]{A 1st Mathematical Primer for the universe of chimpmunks}}

\texttt{\compactlabel[11]{A 1st Mathematical Primer for the universe of chimpmunks}}

\texttt{\compactlabel[12]{A 1st Mathematical Primer for the universe of chimpmunks}}

\texttt{\compactlabel[13]{A 1st Mathematical Primer for the universe of chimpmunks}}

\end{document}