/\A((?:19|20)[0-9]{2})[\- \/.](0[1-9]|1[012])[\- \/.](0[1-9]|[12][0-9]|3[01])\z/

require 're'

include Re

delim                = re.any("- /.")
century_prefix       = re("19") | re("20")
under_ten            = re("0") + re.any("1-9")
ten_to_twelve        = re("1") + re.any("012")
ten_and_under_thirty = re.any("12") + re.any("0-9")
thirties             = re("3") + re.any("01")

year = (century_prefix + re.digit.repeat(2)).capture(:year)
month = (under_ten | ten_to_twelve).capture(:month)
day = (under_ten | ten_and_under_thirty | thirties).capture(:day)

date = (year + delim + month + delim + day).all

result = date.match("2009-01-23")
result[:year]      # => "2009"
result[:month]     # => "01"
result[:day]       # => "23"

PHONE_RE = re.digit.repeat(3).capture(:area) +
               re("-") +
               re.digit.repeat(3).capture(:exchange) +
               re("-") +
               re.digit.repeat(4)).capture(:subscriber)

PHONE_REGEXP = PHONE_RE.regexp

if PHONE_REGEXP =~ string
  # blah blah blah
end