i think i found a little error in the regular expression howto (http://docs.python.org/py3k/howto/regex.html).
Concerning \w, the howto states:
\w Matches any alphanumeric character; this is equivalent to the class [a-zA-Z0-9_].
However, \w is *not* equivalent to [a-zA-Z0-9_]. For example, the latter does not accept letterw with accents àéîü, whereas the former does.