Fetch the next token by trying to match any of the regexes in order.

def NextToken(self):
    """Fetch the next token by trying to match any of the regexes in order."""
    # Nothing in the input stream - no token can match.
    if not self.buffer:
      return

    current_state = self.state
    for token in self._tokens:
      # Does the rule apply to us?
      if token.state_regex and not token.state_regex.match(current_state):
        continue

      if self.verbose:
        logging.debug("%s: Trying to match %r with %r", self.state,
                      self.buffer[:10], token.re_str)

      # Try to match the rule
      m = token.regex.match(self.buffer)
      if not m:
        continue

      if self.verbose:
        logging.debug("%s matched %s", token.re_str, m.group(0))

      # A token matched the empty string. We can not consume the token from the
      # input stream.
      if m.end() == 0:
        raise RuntimeError("Lexer bug! Token can not match the empty string.")

      # The match consumes the data off the buffer (the handler can put it back
      # if it likes)
      self.processed_buffer += self.buffer[:m.end()]
      self.buffer = self.buffer[m.end():]
      self.processed += m.end()

      next_state = token.next_state
      for action in token.actions:
        if self.verbose:
          logging.debug("Calling %s with %s", action, m.group(0))

        # Is there a callback to handle this action?
        cb = getattr(self, action, self.Default)

        # Allow a callback to skip other callbacks.
        try:
          possible_next_state = cb(string=m.group(0), match=m)
          if possible_next_state == "CONTINUE":
            continue
          # Override the state from the Token
          elif possible_next_state:
            next_state = possible_next_state
        except ParseError as e:
          self.Error(e)

      # Update the next state
      if next_state:
        self.state = next_state

      return token

    # Check that we are making progress - if we are too full, we assume we are
    # stuck.
    self.Error("Lexer stuck at state %s" % (self.state))
    self.processed_buffer += self.buffer[:1]
    self.buffer = self.buffer[1:]
    return "Error"