\b(?:Jan(?:uary)?|Feb(?:ruary)?|...|Dec(?:ember)?) (?:19[7-9]\d|2\d{3})(?=\D|$)

那是

\b                  # a word boundary
(?:                 # non-capturing group
  Jan(?:uary)?      # Jan(uary)
  |Feb(?:ruary)?    #
  |...              # and so on
  |Dec(?:ember)?    # Dec(ember)
)                   # end group
                    # a space
(?:                 # non-capturing group
  19[7-9]\d|2\d{3}  # 1970-2999
)                   # end group
(?=\D|$)            # followed by: anything but a digit or the end of string