$ irb -r enumerator
irb(main):001:0> "12345678".split(//).enum_slice( 2 ).inject( [] ) { |a,x| a << x.join( "" ) }.join( " " ) 
=> "12 34 56 78"
irb(main):002:0> "123456789".split(//).enum_slice( 2 ).inject( [] ) { |a,x| a << x.join( "" ) }.join( " " )
=> "12 34 56 78 9"