Nothing. The three nested results correspond to the three regexes nested in combinedReg: numReg, holidayName, and holidayDateDetails. None of those regexes has other regexes nested within them.

Are you confusing nested regexes with marked sub-expressions? Perhaps if you change your innermost nested FOREACH statement with something like this:

 BOOST_FOREACH (const ssub_match& ssub, res)
         cout << '\t'  << ssub << endl;

... you'll get something like what you were expecting.