Tcl docs (re_syntax.n) say that the "CLASS-SHORTHAND ESCAPES"
are defined:

 \w        [[:alnum:]_] (note underscore)

By this description, \u203f ought to match
regexp {\w} only if it matches {[[:alnum:]]}.

% info patch
8.6.6
% regexp {\w} \u203f
1
% regexp {[[:alnum:]]} \u203f
0

Something in this collection of facts isn't right.