"Let \mu be a finite measure on the Borel sigma-algebra \mathfrak{B}(X) of a metric space X.

Prove that the class \mathfrak{B} of all Borel sets that are both inner and outer regular is a sigma algebra. Deduce that every Borel set is inner regular."

I have a hard time visualizing what the complement of the inner/outer regular set is and how it all fits together. Any help is appreciated. (I think this is #12 in chapter 2 or Pollard's prob text)