Modify Cache-Control Header

Here is the rule I have so far. But I need it to be something like "max-age > 0"

 

when HTTP_RESPONSE {

 

 

if { [HTTP::header value "Cache-Control"] contains "max-age" } {

 

set cache_control "public,max-age=900"

 

HTTP::header replace "Cache-Control" $cache_control

 

}

 

}

 

 

Any ideas?

 

 

TY