"A word is a sequence of letters not containing any other symbols or
ends of line.
The end of a sentence is defined a full stop, a question-mark or an
exclamation mark. "

What means "a full stop?" It seems to be EOF, but it isn't true.

I've got AC assuming that "a full stop" was a dot ('.')

Full stop means the punctuation sign '.' in English :)
check your English dictionary