LaTeX fix for Jekyll

    """Replace $..$ by `$..$` and $$..$$ by `$$..$$`, but
        * Don't replace \$ (\ should escape)
        * Don't replace if one of the patterns above is in code blocks:
            ```
            ....
            ...   $...$
            ```
          or
            {% highlight [some language] %}
            ...
            ...  $...$
            {% endhighlight %}
    """
    import re
    # first two dollar signs environment
    content = re.sub(r'(?<![\\])\$\$([^\$]+)\$\$', "`\n$$\g<1>$$`", content)
    # then one dollar sign environment
    content = re.sub(r'(?<!\<span\>)(?<!\$)\$([^\$]+)\$', "`$\g<1>$`", content)