From: ara.t.howard@...
Date: 2006-06-06T00:31:13+09:00
Subject: Re: parsing a boolean expression

On Mon, 5 Jun 2006, Daniel Baird wrote:

> Hmm.. I haven't done anything with yacc before, and my expressions _are_
> going to be pretty simple, so I guess your advice is to write it myself :)
>
> I'd kinda fancied returning a tree made up of nodes that are either:
>
> - a token
> - a list of nodes with a common operator
>
> eg "A and ( B or C or D) and E" would be:
>
> node0: AND(A, node1, E)
> node1: OR(B, C, D)
>
> It's not a binary tree but it would be somewhat convenient for the
> processing / storing / re-displaying I need to do.. I just hoped I'd save a
> day or two standing on someone else's shoulders.
>
> Thanks Matthew.
>
> ;D

this is extremely short and relatively secure, it may fit depending on your use
case and leverages the fact that ruby knows how to parse ruby:

     harp:~ > cat a.rb
     class BoolExp
       attr 'exp'
       attr 'value'

       def initialize exp, context = {}
         @exp = exp.to_s

         tokens = context.keys.map{|k| k.to_s}.join '|'
         re = %r/^(?:\s*|#{ tokens }|[)(]|and|or|not)+$/i

         raise "bad exp <#{ @exp }>" unless @exp =~ re

         @value = Thread.new(exp, context) do |e, c|
           $SAFE = 4
           Module.new do
             sc =
               class << self
                 self
               end
             c.each do |k,v|
               case k.to_s
                 when %r/^[A-Z]/
                   const_set k, v
                 else
                   sc.module_eval{ attr_accessor k }
                   send "#{ k }=", v
               end
             end
             break module_eval(e)
           end
         end.value
       end
     end


     context = {:A => true, :B => false, :C => true, :D => false, :E => true}

     be = BoolExp.new "A and ( B or C or D) and E", context
     p be.value

     be = BoolExp.new "fork", context
     p be.value



     harp:~ > ruby a.rb
     true
     a.rb:11:in `initialize': bad exp <fork> (RuntimeError)
             from a.rb:41



regards.

-a
-- 
suffering increases your inner strength.  also, the wishing for suffering
makes the suffering disappear.
- h.h. the 14th dali lama