Extract data from xml file

Hello,

I want to dynamically extract information from an xml file, something like this:

<bookmarket> <book> <title>T1</title> </book> <book> <title>T2</title> </book> </bookmarket>

I've created a filter:

filter {

xml {
source => "message"
target => "source"
store_xml => false
remove_namespaces => true
xpath => [
#here I don't know the number of books in the xml file
"count(bookstore/step)","number_of_books"
#I want to create something like this:
for counter in [1.. %{number_of_books}] {
"bookmarket/book[counter]/title/text()", "bookTitle[counter]"
}
]
force_array => false
}
}

Thank you for your help.

This seems more like an XPath question that's probably better asked where XPath experts hang out.