ruby1.8 case-insensitive matches fail when using hex escapes

This must be a bug, all the following regular expressions should be
equivalent, but aren't:

% /usr/local/svn-ruby-1.8/bin/ruby -e 's="\x62".upcase; p(s =~ /\x42/i); p(s =~ /\x62/i); p(s =~ /#{"\x62"}/i); p(s =~ /[\x62]/i);'
0
nil
0
0

They are equivalent in ruby1.9:

sroberts@pebble:~/s/ruby/svn-ruby-trunk% /usr/local/svn-ruby-trunk/bin/ruby -e 's="\x62".upcase; p(s =~ /\x42/i); p(s =~ /\x62/i); p(s =~ /#{"\x62"}/i); p(s =~ /[\x62]/i);'
0
0
0
0

sroberts@pebble:~/s/ruby/svn-ruby-trunk% /usr/local/svn-ruby-trunk/bin/ruby --version                                                                        ruby 1.9.0 (2007-02-28 patchlevel 0) [i686-linux]
sroberts@pebble:~/s/ruby/svn-ruby-trunk% /usr/local/svn-ruby-1.8/bin/ruby --version
ruby 1.8.6 (2007-02-28 patchlevel 5000) [i686-linux]