Question

$ gem -v
1.8.5
$ ruby -v
ruby 1.8.7 (2010-04-19 patchlevel 253) [x86_64-linux], MBARI 0x6770, Ruby Enterprise Edition 2010.02

$ sudo gem install sanitize
ERROR:  Error installing sanitize:
    nokogiri requires Ruby version >= 1.8.7

This happens for other gems as well, not just nokogiri. I am on Ruby 1.8.7 and have set my $PATH to include /opt/ruby-enterprise-1.8.7-2010.02/bin/ruby before /usr/bin where system Ruby (1.8.6) is installed.

$ gem env
RubyGems Environment:
  - RUBYGEMS VERSION: 1.8.5
  - RUBY VERSION: 1.8.7 (2010-04-19 patchlevel 253) [x86_64-linux]
  - INSTALLATION DIRECTORY: /opt/ruby-enterprise-1.8.7-2010.02/lib/ruby/gems/1.8
  - RUBY EXECUTABLE: /opt/ruby-enterprise-1.8.7-2010.02/bin/ruby
  - EXECUTABLE DIRECTORY: /opt/ruby-enterprise-1.8.7-2010.02/bin

$ which ruby
/opt/ruby-enterprise-1.8.7-2010.02/bin/ruby
$which gem
/opt/ruby-enterprise-1.8.7-2010.02/bin/gem

Answer 1

Your sudo command is most probably changing your PATH variable. Try sudo gem env and you can verify this. As a solution you might try this:

sudo env PATH=$PATH gem install ...

Also see here for further discussion:
sudo changes PATH – why?

As a workaround you could also do:

sudo su -m

To get to a root prompt using the -m modifier to preserve environment. Then from that root prompt do your gem installs.