ruby 1.9 regex to match s-expressions

gistfile1.rb

#matches something like an s-expression
R = %r@
      (?<atom> [-\w]+){0}
      (?<quote> '\g<expr>){0}
      (?<expr>  
        \g<atom> | 
        \g<quote> | 
        \(  
          \g<expr> 
          (?<exprs> (\s+  \g<expr>)* )
        \)
      ){0}
      ^\g<expr>$
    @imx
def t s
  p '='*30
  p s
  p R.match(s)
  p s.scan R
end
t('(ciao)')
t('(ciao miao)')
t('(ciao miao)')
t('(ciao (miao))')
t('(ciao (miao bau))')
t("(ciao '(miao bau))")

t('(sum-iter k 1)')
t(<<SEXPR)
(define sum1  (lambda (k)
    (sum-iter k 0 1)))
SEXPR

t("ciao '(miao bau))")
t('(ciao')
t(<<SEXPR)
(define sum1  (lambda (k)
    (sum-iter k 0 1))
SEXPR