如何自定义php-markdown链接语法

Links can be defined through:

[foo](http://foo)
//or
[foo][1]

How do you customize php-markdown so you can choose whether it would be rel="nofollow" or not? Something like

[foo](!http://foo "this is the title")

where ! signifies the a rel="nofollow" attribution?

For reference links, modify the lines around line 733 (at the time of writing) to add nofollow if appropriate. For inline links, modify line 752.

The reference link code currently looks like this:

$url = $this->urls[$link_id];
$url = $this->encodeAttribute($url);

$result = "<a href=\"$url\"";
if ( isset( $this->titles[$link_id] ) ) {
    $title = $this->titles[$link_id];
    $title = $this->encodeAttribute($title);
    $result .=  " title=\"$title\"";
}

$link_text = $this->runSpanGamut($link_text);
$result .= ">$link_text</a>";
$result = $this->hashPart($result);

The inline link code looks similar. You'll want to add a check for if $url begins with ! and if so, strip it and put rel="nofollow" at the right place in $result.