Answers to the question linked in domwass's comment mention the package
url. This package is already loaded by
biblatex. It is configured with the command
\biburlsetup defined in
URLs are broken at the set of characters specified in
\UrlBigBreaks will prevent breaks from occurring between repeating characters (e.g.
:: in the code below).
\UrlSpecials handles breaks at characters that may not be present in the document font.
Upgrading to the latest version of
biblatex should solve your problem as
\biburlsetup was expanded in version
1.4c to permit breakpoints at many different characters. For reference here is the definition for
\biburlsetup from version
\Urlmuskip=0mu plus 3mu\relax
1.6, this new definition no longer allows breakpoints at numbers, uppercase letters and lowercase letters by default. Breaks at these characters can be permitted by setting the penalty counters to a value between
10000, exclusive. For example:
The penalties can be reset locally using the command
\defcounter from the
etoolbox package (also loaded by
biblatex). For example:
To use this new setup with an older version of
biblatex, just put the code defining
\biburlsetup and its penalty counters in your preamble and replace