conv_exs.rb

#!/usr/bin/env ruby

(inpath, outpath, org, dst = $*)

unless inpath && outpath && org && dst
  puts "usage: conv_exs infile outfile org dst"
  exit 1
end

binstr = nil
open(inpath, "rb") do |file|
  binstr = file.read
end

pos = 0
loop do
  idx = binstr.index(org, pos)
  break unless idx
  newstr = binstr[idx, 256].gsub(org, dst).rstrip.ljust(256, "\0")
  binstr[idx, 256] = newstr
  pos = idx + 1
end

open(outpath, "wb") do |file|
  file.write(binstr)
end