I don't understand how bIsValidChar || oEvent.ctrlKey returns false for pressing the ctrl key. Won't hitting the ctlr key return false for bIsValide (null) then with the || (or) go to oEvent.ctrlKey which will return true?

You don't want to block the default behavior when the Ctrl key is pressed because it will interfere with normal textbox operations. So if the character is valid or if Ctrl is pressed, the function returns true so the default behavior continues. (bIsValidChar || event.ctrlKey) will never return false when the Ctrl key is pressed because that would block the default behavior.

Nicholas C. Zakas
Author, Professional JavaScript for Web Developers (ISBN 0764579088)
http://www.nczonline.net/