class Rouge::RegexLexer::Rule

@see StateDSL#rule
to perform if the test succeeds.
A rule is a tuple of a regular expression to test, and a callback

def beginning_of_line?

calling Regexp#source more than once.
Since Regexps are immuntable, this is cached to avoid

Does the regex start with a ^? 
def beginning_of_line?
  return @beginning_of_line if instance_variable_defined?(:@beginning_of_line)
  @beginning_of_line = re.source[0] == ?^
end

def initialize(re, callback) 

def initialize(re, callback)
  @re = re
  @callback = callback
end

def inspect 

def inspect
  "#<Rule #{@re.inspect}>"
end