Cite like [1-2] instead of [1, 2]

Question

I'm using natbib package with compress option, which works well in the case [1-3]. However, I hope that [1,2] could also be compressed as [1-2].

Answer 1

The built-in mechanism in natbib does not make range for two numbers. To do that, we have to patch the code:

\begin{filecontents}{\jobname.bib}
@article{demo1,
  title   = {Some things {I} did},
  author  = {Other, A. N.},
  journal = {J. Irrep. Res.},
  year    = {2012},
  pages   = {x-y}
}

@article{demo2,
  title   = {Cool!},
  author  = {Nobacon, D.},
  journal = {Ann. Improb. Res.},
  year    = {2012},
  pages   = {a-b}
}
\end{filecontents}
\documentclass{article}
\usepackage[numbers,sort&compress]{natbib}
\usepackage{etoolbox}
\makeatletter
\patchcmd{\NAT@citexnum}
  {%
    \ifx\NAT@last@yr\relax
      \def@NAT@last@yr{\@citea}%
    \else
      \def@NAT@last@yr{--\NAT@penalty}%
    \fi
  }
  {%
    \def@NAT@last@yr{--\NAT@penalty}%
  }
  {}{\FAIL}
\makeatother
\bibliographystyle{unsrtnat}
\begin{document}
\cite{demo1,demo2}
\bibliography{\jobname}
\end{document}

All I'm doing there is taking out the test which seems to generate the comma (using \@citea): natbib is very careful not to use too many macros and registers, but that makes the code a bit tricky to read!