class Regexp::Parser

def open_group(token)
  case token.token
  when :passive
    exp = Group::Passive.new(token, active_opts)
  when :atomic
    exp = Group::Atomic.new(token, active_opts)
  when :named
    exp = Group::Named.new(token, active_opts)
  when :capture
    exp = Group::Capture.new(token, active_opts)
  when :absence
    exp = Group::Absence.new(token, active_opts)
  when :lookahead
    exp = Assertion::Lookahead.new(token, active_opts)
  when :nlookahead
    exp = Assertion::NegativeLookahead.new(token, active_opts)
  when :lookbehind
    exp = Assertion::Lookbehind.new(token, active_opts)
  when :nlookbehind
    exp = Assertion::NegativeLookbehind.new(token, active_opts)
  else
    raise UnknownTokenError.new('Group type open', token)
  end
  # Push the active options to the stack again. This way we can simply pop the
  # stack for any group we close, no matter if it had its own options or not.
  options_stack << active_opts
  nest(exp)
end