Looking for a website about Independents/Libertarians voting in primaries

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I was under the impression that only registered republicans could vote for republicans and registered democrats for democrats.

Now, it looks like some states are letting republicans vote for democrats and vice versa as well as Independents can vote for either party.

I'm a registered republican, but don't feel like wasting a vote on Ron Paul. Anyone know what the rules are in NC, google failed me?

Only thing I found out about NC is it is a semi-open primary