Regular expressions "[\u]" and "\u", unlike "[[:upper:]]", will not match any letters when the icase flag is set. Similarly, "\U" will match anything. Expressions "\l" and "\L" seem to be fine due to the lower_case_map[] translation.
My suggestion would be to replace
classes.push(traits_type::char_class_upper);
and the like by
classes.push((_flags & regex_constants::icase) ? traits_type::char_class_alpha : traits_type::char_class_upper);
in the 'syntax_u/U/l/L' cases in set_compile() and set_expression(). This approach is currently taken in the 'syntax_open_set' case in set_compile().
John may have a different fix in mind, though. :-)
Thanks, I'm actually in the process of rewriting all the expression parsing code, so yes the fix will be different, although for the time being, I'll commit the fix you suggested. John.