Looking for patterns matching numeric ranges with regex

match_range_code_spike.rb

module Match
  module_function

  def range_upto n
    s = n.to_s.freeze

    ones = s[-1].to_i
    tens = s[-2].to_i
    hundreds = s[-3].to_i

    if n < 0
      raise NotImplementedError, 'Negatives :O'
    elsif n == 0
      /\A0\z/
    elsif n <= 9
      /\A[0-#{n}]\z/
    elsif n <= 19
      /\A(1[0-#{ones}]|\d)\z/
    elsif n <= 99
      /\A(#{tens}[0-#{ones}]|[1-#{tens.pred}]?\d)\z/
    elsif n <= 199
      if tens.zero?
        /\A(1[0-#{tens}][0-#{ones}]|[1-9]?\d)\z/
      else
        /\A(1[0-#{tens}][0-#{ones}]|1[0-#{tens.pred}]\d|[1-9]?\d)\z/
      end
    elsif n <= 999
      if tens.zero?
        /\A([1-#{hundreds}][0-#{tens}][0-#{ones}]|[1-#{hundreds.pred}]\d{2}|[1-9]?\d)\z/
      else
        /\A([1-#{hundreds}][0-#{tens}][0-#{ones}]|#{hundreds}[0-#{tens.pred}]\d|[1-#{hundreds.pred}]\d{2}|[1-9]?\d)\z/
      end
    else
      raise NotImplementedError, 'More than three digits :O'
    end
  end
end

require 'minitest/autorun'

describe Match do
  it 'accepts upto the max' do
    0.upto 999 do |n|
      r = Match.range_upto(n)
      0.upto n do |x|
        assert x.to_s =~ r, "#{r.inspect} for `#{n}` doesn't match \"#{x}\""
      end
    end
  end

  it 'rejects one over the max' do
    0.upto 999 do |n|
      r = Match.range_upto(n)
      assert n.next.to_s !~ r, "#{r.inspect} matches \"#{n.next}\" when `#{n}` is max"
    end
  end
end