I wrote this mostly to annoy jeffa because of:

they could be combined into one regex

But, he insisted that I post this here :)

$chunk =~
s!
    \[
    (?(?= [^:]*://)
        (?:
            (\w+)://
            ([^|\]]+)
            (?:
                \| ([^\[]+)
            )?
        )
        |
        (\w+)
    )
    \]
!
$_ = "<a href=\"".((defined$1)?(qq($TAG{$1}$2">).((defined$3)?$3:$2)):
qq($TAG{DEFAULT}$4">$4))."</a>"!gex;
[download]

I truely think that jeffa's approach is better than the above. It's much smarter to break a 3 case problem into 3 steps rather than use 1 gigantic regex. Just look at the "substitution" section; its hidious (I'd normally have used a sub to handle the above "substitution" section, but then wouldn't be "one regex" :)

.