https://developer-old.gnome.org/glib/stable/glib-regex-syntax.html

Thanks ... but I was hoping to have my homework done form me ;-)

"Match whole word", "Match from start of word" and "Use escape
sequences" do not apply to regexes, so those settings are made
insensitive (greyed out) when regex is set.  Insensitive UI items
cannot be changed which is why it behaves the way you observed.  It is
not a bug, its the way the toolkit works.

Yeah, I know the "Match ... " is incompatible, but I was expecting it to wink out, or something.  Looking closely I see what you mean, the tick turns subtly grey but it's really hard to notice.  Again, it's hardly important, but maybe if the whole icon turned red or something it would be noticeable.  Nevermind! 

Oh, and I did my homework :-) 

Thanks Lex.