Best lexer/parser for Ruby language itself

Hi, all.

I need to do following thing: I have some program on ruby and I would
parse this program, do some manipulations with code and write back it to

Manipulations with sourcecode quite simple, like changing from
puts 2 if a.nil?
if a.nil?
puts 2

or %Q{hello world} -> “hello world”

program is very similar to ruby code beautifier (and probably will be
somewhere in the future).

I am looking for articles/documents with theory background in this area
well as example programs (better on ruby)

Could you also help me with choosing what is the best lib for doing
that. I
saw bunch of ruby parser/lexer libs among them: grammar, parsetree,
rubylexer, syntax, ripper. I dont know what is the best for my task. And
dont know from what to start. Could you give me any advice on that?