Problem matching accented chars on OS X


I'm finding words within strings in Western European languages, so I need to account
for accented characters, such as ê (e circumflex) and à (a grave). On ruby 1.8.2
MSW the following works for me (simplified):

WORD_PATTERN = /^[\w\xC0-\xD6\xD8-\xF6\xF8-\xFF]+$/s

\w gets me a-z + A-Z , the hex characters are the positions of the accented characters in
iso-8859-1 encoding. This seems to work, but when I run the same code on OS X, I get

.../lib/weft/backend/sqlite.rb:533: mismatch multibyte code length in char-class range: /^[\w\xC0-\xD6\xD8-\xF6\xF8-\xFF]+$/ (SyntaxError)

Any pointers? I'm not sure what is going wrong.

Is there a library written that can help me matching letter characters (ideally in a
variety of codesets)? [:alpha:] regex class seeemed to be synonymous with \w, which
doesn't match enough.