failed.rb

    regex_string = "\n|" \
                    "<b>|</b>|" \
                    "<b>|<b>|" \
                    "*|*|" \
                    "[^<\n]+"



[hannes] ~/dev/test  % ruby parser.rb
parser.rb:12:in `initialize': target of repeat operator is not specified: / (RegexpError)
|<b>|<\/b>|<b>|<b>|*|*|[^<
]+/m
        from parser.rb:12:in `new'
        from parser.rb:12:in `<class:MarkdownParser>'
        from parser.rb:3:in `<main>'

parser.rb

# Execute: `ruby parser.rb`
# Context: building a textformatter for Prawn that takes markdown
# instead of the usual html elements and formats them inline.
# The regex is part of the original with the HTML replacements
# still in place. I added the second <b> matching (without the /)
# as a first step towards a solution, and this still works.
# All of my attempts to replace the <b> with * in that case
# have failed so far. Some simply don't work as expected,
# others throw an error.

require 'minitest/autorun'

class MarkdownParser
  # @group Extension API

  PARSER_REGEX = begin
    regex_string = "\n|" \
                    "<b>|</b>|" \
                    "<b>|<b>|" \
                    "[^<\n]+"
    Regexp.new(regex_string, Regexp::MULTILINE)
  end

  def self.parse input, regex=PARSER_REGEX
    input.scan(regex)
  end
end

class TestMarkdownParser < Minitest::Test
  def test_parser_1
    string = "foo <b>bold</b> bar"
    assert_equal ["foo ", "<b>", "bold", "</b>", " bar"], MarkdownParser.parse(string)
  end

  def test_parser_2
    string = "foo <b>bold<b> bar"
    assert_equal ["foo ", "<b>", "bold", "<b>", " bar"], MarkdownParser.parse(string)
  end

  def test_parser_3
    string = "foo *bold* bar"
    assert_equal ["foo ", "*", "bold", "*", " bar"], MarkdownParser.parse(string)
  end
end