TeX capacity exceeded while nesting two commands from hyperref and xstring

Question

I have wasted most of my afternoon on this one and can't figure it out. I have prepared an MWE that demonstrates the problem. In my \displaylink command I invoke \href{raw_url}{anchor} (from package hyperref) but I want to build the raw_url with another command, \createurl, which manipulates its argument with one of the xstring utilities.

In the MWE there are two versions of \createurl, one without the command from xstring and one (\createurlx) with it. If you comment out the line labeled "fails", and uncomment the line labelled "works", you see that all three commands (\createurl, \createurlx and \displaylink) work fine. If, however, you comment out the "works" like and uncomment the "fails" line, meaning that from \displaylink you invoke \createurlx which has the call to \StrGobbleLeft from xstring, then things stop working. So it seems to be the nesting of these two commands that's the problem. Why, I have no idea.

BTW my current workaround is that things also work if I put the \StrGobbleLeft inside \displaylink, but I don't want to do that because the \createurlx is a command that's used in other places too. (And of course you realize that these manipulations have been simplified here for the purpose of the MWE---they're somewhat less trivial in the actual document.)

\documentclass{article}
\usepackage{hyperref}
\usepackage{xstring}

\newcommand{\createurl}[1]{
  http://www.foobar.com/documents/#1.pdf
}

\newcommand{\createurlx}[1]{
    \StrGobbleLeft{#1}{1}[\chopped]
    http://www.foobar.com/documents/\chopped.pdf
}

\newcommand{\displaylink}[1]{
  % \href{\createurl{#1}}{\mbox{#1}} % works
  \href{\createurlx{#1}}{\mbox{#1}} % fails
}

\begin{document}
The generated url looks like this: \createurl{my-stuff}.
The chopped url looks like this: \createurlx{my-stuff}.
The calling code wants to display a hyperlink: \displaylink{my-stuff}.
\end{document}

This fails with:

    This is pdfTeX, Version 3.14159265-2.6-1.40.18 (TeX Live 2017/Debian) (preloaded format=pdflatex)
 restricted \write18 enabled.
entering extended mode
(./mwe2.tex
LaTeX2e <2017-04-15>
Babel <3.18> and hyphenation patterns for 84 language(s) loaded.
(/usr/share/texlive/texmf-dist/tex/latex/base/article.cls
Document Class: article 2014/09/29 v1.4h Standard LaTeX document class
(/usr/share/texlive/texmf-dist/tex/latex/base/size10.clo))
(/usr/share/texlive/texmf-dist/tex/latex/hyperref/hyperref.sty
(/usr/share/texlive/texmf-dist/tex/generic/oberdiek/hobsub-hyperref.sty
(/usr/share/texlive/texmf-dist/tex/generic/oberdiek/hobsub-generic.sty))
(/usr/share/texlive/texmf-dist/tex/latex/graphics/keyval.sty)
(/usr/share/texlive/texmf-dist/tex/generic/ifxetex/ifxetex.sty)
(/usr/share/texlive/texmf-dist/tex/latex/oberdiek/auxhook.sty)
(/usr/share/texlive/texmf-dist/tex/latex/oberdiek/kvoptions.sty)
(/usr/share/texlive/texmf-dist/tex/latex/hyperref/pd1enc.def)
(/usr/share/texlive/texmf-dist/tex/latex/latexconfig/hyperref.cfg)
(/usr/share/texlive/texmf-dist/tex/latex/url/url.sty))
(/usr/share/texlive/texmf-dist/tex/latex/hyperref/hpdftex.def
(/usr/share/texlive/texmf-dist/tex/latex/oberdiek/rerunfilecheck.sty))
(/usr/share/texlive/texmf-dist/tex/generic/xstring/xstring.sty
(/usr/share/texlive/texmf-dist/tex/generic/xstring/xstring.tex)) (./mwe2.aux)
(/usr/share/texlive/texmf-dist/tex/latex/hyperref/nameref.sty
(/usr/share/texlive/texmf-dist/tex/generic/oberdiek/gettitlestring.sty))
(./mwe2.out) (./mwe2.out)
! TeX capacity exceeded, sorry [input stack size=5000].
\Hy@href {->\hyper@normalise 
                             \href@ {
l.25 ...o display a hyperlink: \displaylink{stuff}
                                                  .
!  ==> Fatal error occurred, no output PDF file produced!
Transcript written on mwe2.log.

EDIT: as @egreg correctly surmises in a comment, this is not the actual string manipulation I need. The actual one requires transforming an input like 2010-x03-y10 into 2010x3y10, that is to say removing the hyphens and any leading zeros---and then adding a constant prefix and suffix. I have had no trouble coding that up as a sequence of xstring operations and storing the result in a command name (the \chopped of my MWE), but I have not been able to "hand the obtained string to \href" (in plain Latex please if possible---but I'm told by @Herbert below that it's not).

Answer 1

If no expl3 features are to be used, I suggest to apply \StrGobbleLeft in \displaylink and apply \chopped there. The \StrGobbleLeft macro can still be applied in \createurlx anyway.

\StrGobbleLeft is like almost any of the xstring macros not expandable, since it does 'severe' manipulations ('assignments') of the input strings and has optional arguments -- this is not expandable and can as such not occur in \href.

\documentclass{article}
\usepackage{xstring}
\usepackage{hyperref}

\newcommand{\displaylink}[1]{%
  \StrGobbleLeft{#1}{1}[\chopped]%
  \href{http://www.foobar.com/documents/\chopped.pdf}{\mbox{#1}}
}

\begin{document}
The calling code wants to display a hyperlink: \displaylink{my-stuff}.
\end{document}

Answer 2

\StrGobbleLeft can not be used in such arguments. If you really only want to remove one char you can use \tl_tail:n from expl3, this is expandable:

\documentclass{article}
\usepackage{hyperref}
\usepackage{expl3}

\newcommand{\createurl}[1]{
  http://www.foobar.com/documents/#1.pdf
}

\ExplSyntaxOn
\newcommand{\createurlx}[1]
 {
    http://www.foobar.com/documents/\tl_tail:n{#1}.pdf
 }

\ExplSyntaxOff

\newcommand{\displaylink}[1]{%
  \href{\createurlx{#1}}{\mbox{#1}}% 
}

\begin{document}
The generated url looks like this: \createurl{my-stuff}.
The chopped url looks like this: \createurlx{my-stuff}.
The calling code wants to display a hyperlink: \displaylink{my-stuff}.
\end{document}

Answer 3

\documentclass[a4paper]{article}
\usepackage[margin=1cm]{geometry}
\usepackage{hyperref}

\newcommand{\createurl}[1]{%
    http://www.foobar.com/documents/#1.pdf%
}    
\newcommand\createurlx[1]{\createURLX#1!!}
\def\createURLX#1#2!!{%
    http://www.foobar.com/documents/#2.pdf%
}
\newcommand{\displaylink}[1]{%
    \href{\createurlx{#1}}{\mbox{#1}}% 
}

\begin{document}
The generated url looks like this: \createurl{my-stuff}.

The chopped url looks like this: \createurlx{my-stuff}.

The calling code wants to display a hyperlink: \displaylink{my-stuff}.
\end{document}