Dragon book exercise questions

I'm currently trying to work through the "dragon" book on my own, but
I'm having a hard time with quite a few of the exercises, since I
don't have an instructor to ask questions, or to even tell me if I've
done them correctly.


Anybody mind if I throw a few questions here? :)


If not, here's the first one:


Chapter 3, exercise 3.7: Write regular definitions for the following
languages
d) All strings of digits with no repeated digit


I have absolutely no clue how to proceed here.


Actually, that's not totally true. I could write a regular definition
for that language, but the only way I can think of, it would be HUGE.
I mean, really monstrous. So monstrous it might only be slightly
smaller than enumerating each string individually. Which tells me
that I'm missing something.


I don't understand this book at all, by the way. Many texts provide
answers to half of the exercises. Would it really be too much to ask
for this one to have done the same? :(


Is there a better text that is more friendly to somebody who's trying
to pick this stuff up on their own, but still covers all the basics?
[I don't know of any way to write a reasonable RE for that language,
either. I think that's the point of the exercise. See the FAQ for
reviews of other books. -John]

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