Regexp: repeating capture group

regexp.rb

pattern = %r{
        \A
        \s*(?:Every)?\s*
        (?:
          (?<monthweek>1st|2nd|3rd|4th|last|first|second|third|fourth)
          (?:
            \s*(?:and|&|,)\s*
            (?<monthweek>1st|2nd|3rd|4th|last|first|second|third|fourth)
          ){0,3}
        )?\s*
        (?<weekday>(?:Sun|Mon|Tues|Wednes|Thurs|Fri|Satur)day)s?\s*
        (?:of each month|of the month|every month)?\s*
        \z
}xi

pattern.match "first and second and third monday"

# #<MatchData
#  "first and second and third monday"
#  monthweek:"first"
#  monthweek:"third"
#  weekday:"monday">

# What happened to monthweek: "second"?