fake_regexp.rb

require 'active_support/all'

class FakeRegexp
  def initialize(pattern)
    @pattern = pattern
  end

  def matches?(str)
    case
    when pattern.empty?
      str.empty?
    when head_char_star?
      with_head_char_repeated.matches?(str) || tail.matches?(str)
    when head_char_matches?(str)
      tail.matches?(str[1..-1])
    else
      false
    end
  end

  def to_s
    pattern
  end

  def inspect
    "/#{pattern}/"
  end

  protected

  attr_reader :pattern

  def head_char
    @head_char ||= pattern[0]
  end

  def head_char_matches?(str)
    (head_char == '.' && !str.empty?) || head_char == str[0]
  end

  def head_char_star?
    pattern[1] == '*'
  end

  def with_head_char_repeated
    self.class.new("#{head_char}#{pattern}")
  end

  def tail
    head_length = head_char_star? ? 2 : 1
    @tail ||= self.class.new(pattern[head_length..-1])
  end
end

test_fake_regexp.rb

def assert_match(pattern, string)
  regexp = FakeRegexp.new(pattern)
  STDERR.puts "#{regexp.inspect} does not match #{string.inspect}" unless regexp.matches?(string)
end

def assert_no_match(pattern, string)
  regexp = FakeRegexp.new(pattern)
  STDERR.puts "#{regexp.inspect} matches #{string.inspect}" if regexp.matches?(string)
end

assert_match 'a', 'a'
assert_match 'aa', 'aa'
assert_match '', ''
assert_match '.', 'a'
assert_match '..', 'aa'
assert_match 'a.', 'ab'
assert_match '.a', 'ba'
assert_match 'a.c', 'abc'
assert_match 'a*', ''
assert_match 'a*', 'a'
assert_match 'a*', 'aaa'
assert_match 'ab*', 'ab'
assert_match 'ab*', 'a'
assert_match 'a*a', 'a'
assert_match 'a*a', 'aa'
assert_match 'a*a', 'aaa'
assert_match 'a*b*', ''
assert_match 'a*b*', 'a'
assert_match 'a*b*', 'b'
assert_match 'a*b*', 'aa'
assert_match 'a*b*', 'ab'
assert_match 'a*b*', 'bb'
assert_match 'a*b*', 'aabb'

assert_no_match 'a', 'aa'
assert_no_match 'aa', 'a'
assert_no_match '', 'a'
assert_no_match 'a', ''
assert_no_match '.', ''
assert_no_match 'a.', ''
assert_no_match 'a.', 'abc'
assert_no_match 'a.', 'ba'
assert_no_match '.a', 'ab'
assert_no_match 'a.c', 'ac'
assert_no_match 'a.c', 'acb'
assert_no_match 'a*', 'b'
assert_no_match 'a*', 'ba'
assert_no_match 'a*', 'aaab'
assert_no_match 'ab*', 'aba'
assert_no_match 'ab*', 'ba'
assert_no_match 'a*a', ''
assert_no_match 'a*b*', 'ba'
assert_no_match 'a*b*', 'abba'