Manually breaking a long URL (in the bibliography)

Question

I have a large bibliography in which a few items are causing underfull hboxes due to a long URL. Answers to similar questions suggest a number of global options for fixing this; e.g.

Forcing linebreaks in \url

hyperref url breaking

Line breaking of URLs in bibliographies

However, I don't want a global solution (like applying \sloppy or \raggedright to the whole bibliography), because all the other bibliography items behave as I want them to.

In most cases, I have been able to resolve the issue by inserting a manual line break at the start of the url entry in the .bib file. However, in one case that isn't working. MWE:

\documentclass[natbib]{svmono}
\usepackage{url}

\begin{document}
\cite{koepke2009naturalness}
\bibliographystyle{harvard}
\bibliography{test}
\end{document}

test.bib

@article{koepke2009naturalness,
  year={2009},
  title={Naturalness in formal mathematics},
  journal={\\\url{http://www.math.uni-bonn.de/people/koepke/Preprints/Naturalness_in_formal_mathematics.pdf}},
  author={Koepke, Peter}
}

I've tried inserting \\ at various points inside the url; it breaks the line, but I still get an underfull hbox. How can I break the url in a way which doesn't cause an underfull hbox?

Edit: http://www.springer.com/computer/lncs?SGWID=0-164-6-793341-0 <-- svmono http://iccle.googlecode.com/svn-history/r2130/trunk/doc/harvard.bst <-- harvard.bst

Answer 1

Update: This question's OP himself has found a good solution between an extra \\ and a working hyperref.

With the help of the hyperref package one can write (the example is changed to an unpublished publication because there is, apparently, no journal):

@unpublished{koepke2009naturalness,
  year={2009},
  title={Naturalness in formal mathematics},
  note={\\
        \href{http://www.math.uni-bonn.de/people/koepke/Preprints/Naturalness_in_formal_mathematics.pdf}
             {\nolinkurl{http://www.math.uni-bonn.de/people/}}
        \\
        \href{http://www.math.uni-bonn.de/people/koepke/Preprints/Naturalness_in_formal_mathematics.pdf}
             {\nolinkurl{koepke/Preprints/Naturalness_in_formal_mathematics.pdf}}
       },
  author={Koepke, Peter}
}

which gives

If only one \href is used, like

  note={\\
        \href{http://www.math.uni-bonn.de/people/koepke/Preprints/Naturalness_in_formal_mathematics.pdf}
             {\nolinkurl{http://www.math.uni-bonn.de/people/}
             \\
              \nolinkurl{koepke/Preprints/Naturalness_in_formal_mathematics.pdf}
             }
       },

the link-active area spans the whole first line:

Original answer

An underfull hbox doesn't mean that there's necessarily something wrong. It is merely a warning.

Compare the bibliography entries of the following code.
The one with the extra \\ doesn't give a warning (but breaks the hyperlink, by the way), the other one does give a warning, but looks the same.

(The italic setting from the journal field did mess up the spacing either way.)

Code

\documentclass[natbib]{svmono}
\usepackage{url}
\usepackage[showframe,pass]{geometry}
\begin{filecontents}{test.bib}
@article{koepke2009naturalness,
  year={2009},
  title={Naturalness in formal mathematics},
  journal={Same Journal Name},
  note={\\\url{http://www.math.uni-bonn.de/people/koepke/Preprints/Naturalness_in_}\\\url{formal_mathematics.pdf}},
  author={Koepke, Peter}
}
@article{koepke2009naturalnessa,
  year={2009},
  title={Naturalness in formal mathematics},
  journal={Same Journal Name},
  note={\\\url{http://www.math.uni-bonn.de/people/koepke/Preprints/Naturalness_in_formal_mathematics.pdf}},
  author={Koepke, Peter}
}
\end{filecontents}
\begin{document}
\cite{*}
\bibliographystyle{harvard}
\bibliography{test}
\end{document}

Output