REXML::Document.new take a simple string as good doc? - ruby-on-rails

Question

I would like to check if the xml is valid. So, here is my code
require 'rexml/document'
begin
def valid_xml?(xml)
REXML::Document.new(xml)
rescue REXML::ParseException
return nil
end
bad_xml_2=%{aasdasdasd}
if(valid_xml?(bad_xml_2) == nil)
puts("bad xml")
raise "bad xml"
end
puts("good_xml")
rescue Exception => e
puts("exception" + e.message)
end
and it returns good_xml as result. Did I do something wrong? It will return bad_xml if the string is
bad_xml = %{
<tasks>
<pending>
<entry>Grocery Shopping</entry>
<done>
<entry>Dry Cleaning</entry>
</tasks>}

Answer 1

Personally, I'd recommend using Nokogiri, as it's the defacto standard for XML/HTML parsing in Ruby. Using it to parse a malformed document:
require 'nokogiri'
doc = Nokogiri::XML('<xml><foo><bar></xml>')
doc.errors # => [#<Nokogiri::XML::SyntaxError: Opening and ending tag mismatch: bar line 1 and xml>, #<Nokogiri::XML::SyntaxError: Premature end of data in tag foo line 1>, #<Nokogiri::XML::SyntaxError: Premature end of data in tag xml line 1>]
If I parse a document that is well-formed:
doc = Nokogiri::XML('<xml><foo/><bar/></xml>')
doc.errors # => []

Answer 2

REXML treats a simple string as a valid XML with no root node:
xml = REXML::Document.new('aasdasdasd')
# => <UNDEFINED> ... </>
It does not however treat illegal XML (with mismatching tags, for example) as a valid XML, and throws an exception.
REXML::Document.new(bad_xml)
# REXML::ParseException: #<REXML::ParseException: Missing end tag for 'done' (got "tasks")
It is missing an end-tag to <done> - so it is not valid.