From: Daniel Martin <martin@...>
Date: 2006-07-13T23:19:45+09:00
Subject: Re: Help with a regexp

Daniel Schierbeck <daniel.schierbeck@gmail.com> writes:

> I'm trying to write a regular expression that matches bencoded
> strings, i.e. strings on the form x:y, where x is the numeric length
> of y.
>
> This is valid:
>
>   6:foobar
>
> while this is not:
>
>   4:foo

I don't think that what you want to do is possible with a mere regular
expression.

It might be possible using perl's special
evaluate-code-while-in-regexp (??{ code }) feature, but not with any
language that doesn't allow regular expression evaluations to escape
back into the host language.

The problem is that you want to leave crucial portions of the regexp
uncompiled until the moment that half of the regular expression has
matched, and this is not possible.

But matching bencoded data isn't that hard; here's something I just
whipped up that should handle bencoded data:

require 'strscan'

class BencodeScanner
  def BencodeScanner.scan(str)
    scan = StringScanner.new(str)
    r = BencodeScanner.doscan_internal(scan,false)
    raise "Malformed Bencoded String" unless scan.eos?
    r
  end
  
  private
  
  @@string_regexps = Hash.new {|h,k| h[k] = /:.{#{k}}/m}
  
  def BencodeScanner.doscan_internal(scanner, allow_e=true)
    tok = scanner.scan(/\d+|[ilde]/)
    case tok
      when nil
        raise "Malformed Bencoded String"
      when 'e'
        raise "Malformed Bencoded String" unless allow_e
        return nil
      when 'l'
        retval = []
        while arritem = BencodeScanner.doscan_internal(scanner)
          retval << arritem
        end
        return retval
      when 'd'
        retval = {}
        while key = BencodeScanner.doscan_internal(scanner)
          val = BencodeScanner.doscan_internal(scanner,false)
          retval[key] = val
        end
        return retval
      when 'i'
        raise "Malformed Bencoded String" unless scanner.scan(/-?\d+e/)
        return scanner.matched[0,scanner.matched.length-1].to_i
      else
        raise "Malformed Bencoded String" unless scanner.scan(@@string_regexps[tok])
        return scanner.matched[1,tok.to_i]
    end
  end
end