From: ts <decoux@...>
Date: 2001-06-18T01:19:45+09:00
Subject: [ruby-talk:16567] [TOY] reversed regexp



 ftp://moulon.inra.fr/pub/ruby/rr.tar.gz

 It's a toy and *totally* useless but it's normal because it was inpired by
 a P language :-) 

 It's based on

   http://www.crusoe.net/~jeffp/sexeger/sexeger.html

 The principe is to reverse the regexp and run it against the reversed
 string. For example

   /(\w{2,})(\d)$/.rmatch "+++++++abc12"

 is the same than

  /^(\d)(\w{2,})/.match "21cba+++++++"

 and with the debugging version (rr don't have debug options) this give


pigeon% ruby -rjj -e '/(\w{2,})(\d)$/.rmatch "+++++++abc12"; p $1, $2'
Regexp /(\w{2,})(\d)$/.reverse
  0     begline
  1     start_memory    $1
  2     charset 0-9  (0) 
  3     stop_memory     $1
  4     start_memory    $2
  5     set_number [9] = 32766
  6     set_number [7] = 2
  7     must succeed 2 times then on_failure_jump 10
  8     wordchar
  9     jump 32766 times to 7
 10     stop_memory     $2
 11     end
subexpressions : 2
Fastmap supplied : 0-9

String <<21cba+++++++>> pos=0

  0 begline                      |21cba+++++++            | 
  1 start_memory                 |21cba+++++++            | 
  2 charset                      |21cba+++++++            | 
  3 stop_memory                 2|1cba+++++++             |  $1=2
  4 start_memory                2|1cba+++++++             | 
  5 set_number_at               2|1cba+++++++             | 
  6 set_number_at               2|1cba+++++++             | 
  7 succeed_n                   2|1cba+++++++             |  > 
  8 wordchar                    2|1cba+++++++             | 
  9 jump_n                     21|cba+++++++              | 
  7 succeed_n                  21|cba+++++++              |  > 
  8 wordchar                   21|cba+++++++              | 
  9 jump_n                    21c|ba+++++++               | 
  7 succeed_n                 21c|ba+++++++               |  >10[0]
  8 wordchar                  21c|ba+++++++               | 
  9 jump_n                   21cb|a+++++++                | 
  7 succeed_n                21cb|a+++++++                |  >10[1]
  8 wordchar                 21cb|a+++++++                | 
  9 jump_n                  21cba|+++++++                 | 
  7 succeed_n               21cba|+++++++                 |  >10[2]
  8 wordchar                21cba|+++++++                 |  F10[2]
 10 stop_memory             21cba|+++++++                 |  $2=1cba SUCCESS
"2"
"abc1"
pigeon% 

 If you have, like me, some difficulties to understand how match a regexp,
 it will be *worst* with a reversed regexp :-)


Guy Decoux