Like splinky mentioned, you will have to add some backreferencing parenthesis to your regexes, like this:

$month = '(Jan(?:uary)?|Feb(?:ruary)?|Mar(?:ch)?
              |Apr(?:il)?|May|Jun(?:e)?
              |Jul(?:y)?|Aug(?:ust)?|Sep(?:tember)?
              |Oct(?:ober)?|Nov(?:ember)?|Dec(?:ember)?)\s+';
  $day = '([0-3]?[0-9])(?:th|st|nd|rd)?,?\s+'; 
  $year = '([0-9]{2,4})';
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Note that I removed the ?: from the outermost parenthesis in $month, and added them to $day and $year. Then you could do something like this:

if (( ($month, $day, $year) = (/$month$day$year/io) ) ||
    ( ($day, $month, $year) = (/$day$month$year/io) ) ) {
    $year += 1900 if $year < 100;
    $month = substr($month, 1, 3);
    print "$day-$month-$year\n";
}
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The if statement was copied from splinky's response, but I removed the parenthesis from the regex because they are already in the variables. You may want to fix the $year adjustment to properly handle things.

--ZZamboni