lib/validators/ks_format_validator.rb
class KsFormatValidator < ValidatesRussian::Validator
# see format here: http://ru.wikipedia.org/wiki/Корреспондентский_счёт
validates_using do |ks|
next false unless ks.size == 20
next false unless ks =~ /^\d+$/
next false unless ks[0..2] == '301'
end
end