In Perl regexps, "
\w" matches any "word character": alphanumerics (according to the locale) and underscores. Note that it does not match a whole "word"; just a character of a "word". Also note that English language words do not usually include digits or underscores; these are really the characters legal from the second position in C-style identifiers.
\W" (which unfortunately shares this node) matches any "non-word character" -- the complement of the set matched by "
\w". Thus, it is just "