URI.parse() will not parse a URI containing a non-qualified hostname that begins with a number.  An IRB session:

irb(main):001:0> require 'uri'
=> true

irb(main):002:0> URI.parse("http://threebee")
=> #<URI::HTTP:0x161319e URL:http://threebee>

irb(main):003:0> URI.parse("http://3bee")
URI::InvalidURIError: the scheme http does not accept registry part: 3bee (or bad hostname?)
        from c:/p4/tableau-3-5/workgroup-support/ruby/lib/ruby/1.8/uri/generic.rb:194:in `initialize'
        from c:/p4/tableau-3-5/workgroup-support/ruby/lib/ruby/1.8/uri/http.rb:46:in `initialize'
        from c:/p4/tableau-3-5/workgroup-support/ruby/lib/ruby/1.8/uri/common.rb:484:in `new'
        from c:/p4/tableau-3-5/workgroup-support/ruby/lib/ruby/1.8/uri/common.rb:484:in `parse'
        from (irb):3
        from ♥:0

irb(main):004:0> URI.parse("http://3bee.foo.com")
=> #<URI::HTTP:0x16108fe URL:http://3bee.foo.com>

Note that the qualified form parses correctly, suggesting that the hostname is acceptable.

A discussion of this case can be found in ruby-talk, archived here:

http://blade.nagaokaut.ac.jp/cgi-bin/vframe.rb/ruby/ruby-talk/266700?266598-266714

Date: 2008-01-18 19:12
Sender: Andrew Beers

Note that the following will also fail with an InvalidURIError:

URI.parse("http://xp_workstation:80/foo/bar.js")

I'll alter my patch...

Date: 2007-10-08 16:41
Sender: Andrew Beers

Patch is now attached.

Date: 2007-10-04 17:53
Sender: Andrew Beers

Based on the discussion archived above (also at
http://www.nabble.com/Bug-in-URI.parse--tf4348168.html#a12388384
since the above isn't working today), I am attaching a patch
for two files:

uri/common.rb
and 
test_common.rb

Date: 2007-09-06 18:51
Sender: Andrew Beers

Note that the URI also will not parse when the bare hostname
is a valid local hostname.