C++ :: RegEx - Obtain The Alternation Index?

I was just reviewing some code, and my eye fell on a bit of regex that's intended to parse a date/time stamp into a date and time.

The timestamp uses shorted month names. The regex had all the possible month names to match the entire pattern. If it does, it went through a 2Nd loop to convert the month name into a 1 to 12 numeric value...

This made me wonder, since the regex is already doing the work to verify the alternation, can't it at the same time tell me which of the possible alternations it matched and use that to calculate the numeric value. So basically

Code:
std::regex reMonth("Some more regex stuff here (Jan|Feb|Mar|Apr|May|Jun|Jul|Aug|Sep|Oct|Nov|Dec).");
CString strTest("Some more regex stuff here Aug.");
std::cmatch res;
if (std::regex_match(strTest.GetString(), res, reMonth)) {
//int iMonthNum = res[1].something(); // some code here that returns 8 for Aug.
}

Or is there really no other way out than doing a 2nd level verification to figure out the actual month number. (The real regex is a bit more complex than this).