I guess I find a spell errate here.

[jun-jing]

The highlight in the picture below, It's at Chapter 12 and section's Exercises.
I guess you are trying to write "Submit an invalid email address" instead of "valid".

Anyway thanks again for the wonderful materials.

[jun-jing]

I am sorry for the hurry issue to submit, I guess this is not a submit, it's just my poor English issue...
Sorry for the bother.