I know you have always wondered how would one reverse a string without using loops or any kind of iteration, using, say, regular expressions and recursion. So stop wondering you:

class String
  def r_revert
    sub(/(.+)(.)/){$2 + $1.r_revert}
  end
end
'jablan'.r_revert
#=> 'nalbaj'

or, if you prefer recursive lambdas instead of monkey-patching:

l = ->(s){s.sub(/(.+)(.)/){$2 + l.($1)}}
l.('jablan')
#=> 'nalbaj'