TeX allows line breaking within urls at only a few characters (which are special in urls): ‘&’, ‘.’, ‘#’, ‘?’, and ‘/’ (but not between two ‘/’ characters). A tiny amount of stretchable space is also inserted around these characters to help with line breaking.
For HTML output, modern browsers will also do line breaking within
displayed urls. If you need to allow breaks at other characters you
@/ as needed (see Line Breaks).
By default, in TeX any such breaks at special characters will occur
after the character. Some people prefer such breaks to happen before
the special character. This can be controlled with the
@urefbreakstyle command (this command has effect only in
where the argument how is one of these words:
(the default) Potentially break after the special characters.
Potentially break before the special characters.
Do not consider breaking at the special characters at all; any potential breaks must be manually inserted.