def highlightBlock(self, string):
        """ Highlight a block of text. Reimplemented to highlight selectively.
        """
        if not self.highlighting_on:
            return

        # The input to this function is a unicode string that may contain
        # paragraph break characters, non-breaking spaces, etc. Here we acquire
        # the string as plain text so we can compare it.
        current_block = self.currentBlock()
        string = self._frontend._get_block_plain_text(current_block)

        # Decide whether to check for the regular or continuation prompt.
        if current_block.contains(self._frontend._prompt_pos):
            prompt = self._frontend._prompt
        else:
            prompt = self._frontend._continuation_prompt

        # Only highlight if we can identify a prompt, but make sure not to
        # highlight the prompt.
        if string.startswith(prompt):
            self._current_offset = len(prompt)
            string = string[len(prompt):]
            super(FrontendHighlighter, self).highlightBlock(string)
Highlight a block of text. Reimplemented to highlight selectively.