class Regexp::Lexer

def lex(input, syntax = "ruby/#{RUBY_VERSION}", options: nil, &block)
  syntax = Regexp::Syntax.new(syntax)
  self.tokens = []
  self.nesting = 0
  self.set_nesting = 0
  self.conditional_nesting = 0
  self.shift = 0
  last = nil
  Regexp::Scanner.scan(input, options: options) do |type, token, text, ts, te|
    type, token = *syntax.normalize(type, token)
    syntax.check! type, token
    ascend(type, token)
    if type == :quantifier and last
      break_literal(last)        if last.type == :literal
      break_codepoint_list(last) if last.token == :codepoint_list
    end
    current = Regexp::Token.new(type, token, text, ts + shift, te + shift,
                                nesting, set_nesting, conditional_nesting)
    current = merge_condition(current) if type == :conditional and
      [:condition, :condition_close].include?(token)
    last.next = current if last
    current.previous = last if last
    tokens << current
    last = current
    descend(type, token)
  end
  if block_given?
    tokens.map { |t| block.call(t) }
  else
    tokens
  end
end