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