DouKaku?

def subst_var(file, v0, v1)
  hdocr = '[-]?[\"\'`]?([A-Z]+)'
  shared_reg = 's,;&|=+-*/[](){}'
  former, latter = %w[@$^ .].map{|reg|
    "#{shared_reg}#{reg}".each_char.inject(''){|a,b| "#{a}\\#{b}"}}
  gsub, rev_gsub = [:to_a, :reverse].map{|meth|
    proc{|s| s.gsub(*[v0, v1].__send__(meth))}}
  open(file, 'r'){|f| f.read}.
    gsub(/(?:^#{v0})?(?:[#{former}]#{v0})+[#{latter}]/, &gsub).
    gsub(/".*"/, &rev_gsub).
    gsub(/<<#{hdocr}(?:.|\s)+[^<]#{hdocr}/, &rev_gsub)
end
file = 'sample.rb'
puts subst_var(file, 'v0', 'v1')

open(file, 'w'){|f| f.write(<<EOF)}
v0+v0=v0
@v0 = 0
$v00, ^v0
"v0 v0 "
<<EOS
v0 v0
EOS
$v0, v0, v0
v0/v0*v0-v0+v0
EOF