about_regular_expressions.rb

# -*- coding: utf-8 -*-
# ⬆️unnecessary since ruby 2.0

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?
  end

  # THINK ABOUT IT:
  #
  # We say that the repetition operators above are "greedy."
  #
  # Why?

  # Greedy will consume as much as possible. https://stackoverflow.com/questions/2301285/what-do-lazy-and-greedy-mean-in-the-context-of-regular-expressions

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

  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

  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

  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 # Perl-style regexp match backreferences
    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 (|).
  # [] is designed for chars?
  # https://stackoverflow.com/a/4724613/4053837

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

  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