regex-normalize.xml substitution syntax

[Marek Bachmann]

Hello all,

can someone explain to me, what the $x in the substitution tags in the 
regex-normalize.xml are good for?

e.g.

<regex>
<pattern>([;_]?((?i)l|j|bv_)?((?i)sid|phpsessid|sessionid|cHash)=.*?)(\?|&amp;|#|$)</pattern>
<substitution>$4</substitution>
</regex>

I guess it is some kind of variable but I can't figure out which value 
it could hold.

Thank you very much.