Regex with problem

gistfile1.rb

1) Following regex in ruby is throwing server start (compile) time error on Mac Book Pro while on Ubuntu it is working.
    # Clean all new line character
    string.gsub!(/(?<!\n)\n(?!\n)/, '')

   Error :
    undefined (?...) sequence: /(?<!\n)\n(?!\n)/ (SyntaxError)
   
   Solution :
   string.gsub!(/\r/,"")
   string.gsub!(/\n/," ")


2) named regex in ruby
   s = "/system/books/248/images/1839/medium_thumb/file_name.jpg" 
   m = s.match(/.*\/(?<book_id>\d+)\/.*\/(?<image_id>\d+)\/.*\/.*/)

   #<MatchData "/system/books/248/images/1839/medium_thumb/file_name.jpg" book_id:"248" image_id:"1839"

   Retrive the results:
   book_id  = m[:book_id]   # 248
   image_id = m[:image_id]  # 1839