I'm guessing this is known, but any script I've got that loads rexml 
emits these warnings:

c:/ruby/lib/ruby/site_ruby/1.8/rexml/entity.rb:21: warning: character 
class has
`-' without escape
c:/ruby/lib/ruby/site_ruby/1.8/rexml/entity.rb:21: warning: character 
class has
`-' without escape
c:/ruby/lib/ruby/site_ruby/1.8/rexml/entity.rb:21: warning: character 
class has
`-' without escape
c:/ruby/lib/ruby/site_ruby/1.8/rexml/entity.rb:21: warning: character 
class has
`-' without escape
c:/ruby/lib/ruby/site_ruby/1.8/rexml/doctype.rb:196: warning: regexp has 
`]' wit
hout escape

I've seen this both in 1.8p3 and p4 (from the dm4lab.to mswin bin downloads:

ftp://ftp.ruby-lang.org/pub/ruby/binaries/mswin32/ruby-1.8.0-preview4-i386-mswin32.zip)

-- 

Chris
http://clabs.org/blogki