Here's a patch that will introduce a new variable,
nnmail-split-expert-regexes (boolean, nil by default).

When nnmail-split-expert-regexes is set, every pattern to match in
nnmail-split-fancy and nnimap-split-fancy (which uses the
nnmail-split-fancy functionality) will be surrounded by ".*" front and
back.  I can add a nnimap-split-expert-regexes variable if anyone is
interested.  The effect is to make life easier for those of us who are
used to matching against the full line, not just inside words as
defined by the current syntax table.

Please comment, thanks...

Ted