regex-normalizer.xml substitution value?

[Philip Brown]

I am new to regex. What will the "$1$3" reproduce in the following 
element. What values are $1$3?

<regex>
 <pattern>(.*)(;jsessionid=[a-zA-Z0-9]{32})(.*)</pattern>
 <substitution>$1$3</substitution>
</regex>
if I leave substitution as   <substitution></substitution> will this 
just get rid of ;jsessionid=123456789...

thanks