about_regular_expressions.rb

# -*- coding: utf-8 -*-
require File.expand_path(File.dirname(__FILE__) + '/neo')

class AboutRegularExpressions < Neo::Koan
  def test_a_pattern_is_a_regular_expression
    assert_equal Regexp, /pattern/.class
  end

  def test_a_regexp_can_search_a_string_for_matching_content
    assert_equal "match", "some matching content"[/match/]
  end

  def test_a_failed_match_returns_nil
    assert_equal nil, "some matching content"[/missing/]
  end

  # ------------------------------------------------------------------

  def test_question_mark_means_optional
    assert_equal "ab", "abbcccddddeeeee"[/ab?/]
    assert_equal "a", "abbcccddddeeeee"[/az?/]
  end

  def test_plus_means_one_or_more
    assert_equal "bccc", "abbcccddddeeeee"[/bc+/]
  end

  def test_asterisk_means_zero_or_more
    assert_equal "abb", "abbcccddddeeeee"[/ab*/]
    assert_equal "a", "abbcccddddeeeee"[/az*/]
    assert_equal "", "abbcccddddeeeee"[/z*/]

    # THINK ABOUT IT:
    #
    # When would * fail to match?
    # Possible answer:
    #   When it fails to find any occurence of the pattern, it will return a nil.
  end

  # THINK ABOUT IT:
  #
  # We say that the repetition operators above are "greedy."
  #
  # Why?
  # Answer:
  #   From RubyDocs:
        # Repetition is greedy by default: as many occurrences as possible are matched while still allowing the overall match to succeed. By contrast, lazy matching makes the minimal amount of matches necessary for overall success. A greedy metacharacter can be made lazy by following it with ?.
        # Extra:
        # A quantifier followed by + matches possessively: once it has matched it does not backtrack. They behave like greedy quantifiers, but having matched they refuse to “give up” their match even if this jeopardises the overall match.

  # ------------------------------------------------------------------

  def test_the_left_most_match_wins
    assert_equal "a", "abbccc az"[/az*/]
  end

  # ------------------------------------------------------------------

  def test_character_classes_give_options_for_a_character
    animals = ["cat", "bat", "rat", "zat"]
    assert_equal ["cat", "bat", "rat"], animals.select { |a| a[/[cbr]at/] }
  end

  # Note:
  #   [] in the a[/[]/] is a character-class

  def test_slash_d_is_a_shortcut_for_a_digit_character_class
    assert_equal "42", "the number is 42"[/[0123456789]+/]
    assert_equal "42", "the number is 42"[/\d+/]
  end

  def test_character_classes_can_include_ranges
    assert_equal "42", "the number is 42"[/[0-9]+/]
  end

  def test_slash_s_is_a_shortcut_for_a_whitespace_character_class
    assert_equal " \t\n", "space: \t\n"[/\s+/]
  end

  def test_slash_w_is_a_shortcut_for_a_word_character_class
    # NOTE:  This is more like how a programmer might define a word.
    assert_equal "variable_1", "variable_1 = 42"[/[a-zA-Z0-9_]+/]
    assert_equal "variable_1", "variable_1 = 42"[/\w+/]
  end

  def test_period_is_a_shortcut_for_any_non_newline_character
    assert_equal "abc", "abc\n123"[/a.+/]
  end

  def test_a_character_class_can_be_negated
    assert_equal "the number is ", "the number is 42"[/[^0-9]+/]
  end

  # Note:
  #   ^ skips the pattern following it.

  def test_shortcut_character_classes_are_negated_with_capitals
    assert_equal "the number is ", "the number is 42"[/\D+/]
    assert_equal "space:", "space: \t\n"[/\S+/]
    # ... a programmer would most likely do
    assert_equal " = ", "variable_1 = 42"[/[^a-zA-Z0-9_]+/]
    assert_equal " = ", "variable_1 = 42"[/\W+/]
  end

  # ------------------------------------------------------------------

  def test_slash_a_anchors_to_the_start_of_the_string
    assert_equal "start", "start end"[/\Astart/]
    assert_equal nil, "start end"[/\Aend/]
  end

  def test_slash_z_anchors_to_the_end_of_the_string
    assert_equal "end", "start end"[/end\z/]
    assert_equal nil, "start end"[/start\z/]
  end

  def test_caret_anchors_to_the_start_of_lines
    assert_equal "2", "num 42\n2 lines"[/^\d+/]
  end

  def test_dollar_sign_anchors_to_the_end_of_lines
    assert_equal "42", "2 lines\nnum 42"[/\d+$/]
  end

  def test_slash_b_anchors_to_a_word_boundary
    assert_equal "vines", "bovine vines"[/\bvine./]
  end

  # ------------------------------------------------------------------

  def test_parentheses_group_contents
    assert_equal "hahaha", "ahahaha"[/(ha)+/]
  end

  # ------------------------------------------------------------------

  def test_parentheses_also_capture_matched_content_by_number
    assert_equal "Gray", "Gray, James"[/(\w+), (\w+)/, 1]
    assert_equal "James", "Gray, James"[/(\w+), (\w+)/, 2]
  end

  def test_variables_can_also_be_used_to_access_captures
    assert_equal "Gray, James", "Name:  Gray, James"[/(\w+), (\w+)/]
    assert_equal "Gray", $1
    assert_equal "James", $2
  end

  # ------------------------------------------------------------------

  def test_a_vertical_pipe_means_or
    grays = /(James|Dana|Summer) Gray/
    assert_equal "James Gray", "James Gray"[grays]
    assert_equal "Summer", "Summer Gray"[grays, 1]
    assert_equal nil, "Jim Gray"[grays, 1]
  end

  # THINK ABOUT IT:
  #
  # Explain the difference between a character class ([...]) and alternation (|).
  # Tough one. Answer:
    # From StackOverflow:
    #   Character classes ([...]) are optimized for matching one out of some set of characters, and alternatives (x|y) allow for more general choices of varying lengths. You will tend to see better performance if you keep these design principles in mind. Regex implementations transform source code such as /[abc]/ into finite-state automata, usually NFAs. What we think of as regex engines are more-or-less bookkeepers that assist execution of those target state machines. The sufficiently smart regex compiler will generate the same machine code for equivalent regexes, but this is difficult and expensive in the general case because of the lurking exponential complexity.

  # ------------------------------------------------------------------

  def test_scan_is_like_find_all
    assert_equal ["one", "two", "three"], "one two-three".scan(/\w+/)
  end

  def test_sub_is_like_find_and_replace
    assert_equal "one t-three", "one two-three".sub(/(t\w*)/) { $1[0, 1] }
  end

  def test_gsub_is_like_find_and_replace_all
    assert_equal "one t-t", "one two-three".gsub(/(t\w*)/) { $1[0, 1] }
  end
end