Sometimes you need a quick-and-dirty way of counting some elements, but
not as dirty as calling out to shell and using grep and wc on it
(e.g., grep '<resident>' my.xml | wc -l). Imagine you have the
only have the Ruby standard library available and wish to count the
number of <resident> elements:
require "rexml/document"
xml = <<EOF
<?xml version="1.0" encoding="utf-8"?>
<residents>
<resident name="Foo"/>
<resident name="Bar"/>
<resident name="Baz"/>
</residents>
EOF
doc = REXML::Document.new(xml)
REXML::XPath.first(doc, "count(//residents/resident)")
#=> 3