word delimiters in copy/scrollback mode?

hi screen-users! does anyone know what screen's word delimiters are in
copy/scrollback mode, and if there's any easier way to change them
than in the source?

i'm specifically interested in the W key, which copies the word under
the cursor. i'd like to bind a key to open the URL under the cursor,
using writebuf and the exchange file and exec. unfortunately, URLs
have punctuation in them, so W only copies part of the URL. i'd like
to relax the delimiters to just whitespace, ideally temporarily.

(http://lists.gnu.org/archive/html/screen-users/2005-09/msg00039.html
is related, but assumes marking the URL manually.)