mac="0800273B51A9"

# With reject
mac.split(/(..)/).reject {|c| c.empty?}.join(':')


# With Inject
mac.split(//).inject([]) do |nmac, e|
  if x.nil? or x>0
    nmac[nmac.size-1] = nmac[nmac.size-1] + e
    x=0
  else
    nmac[nmac.size] = e
    x=x+1
  end
  nmac
end.join(':')

mac.split(//).inject([]) { |m, e| if x.nil? or x>0 ;
    m[m.size-1] = m[m.size-1] + e ; x=0 ; else ; m[m.size] = e ;
    x=x+1 ; end ; m }.join(':')