From: gabriele renzi <rff_rff@...>
Date: 2004-11-05T18:53:46+09:00
Subject: Re: recursive brace matching with Ruby regexp

Mark Hubbart ha scritto:

> Hi,
> 
> On Fri, 5 Nov 2004 09:04:30 +0900, Jason Sweat <jason.sweat@gmail.com> wrote:
> 
>>I wanted to learn Ruby, so I picked a small task of trying to write a
>>command line script to parse PHP classes and shell out some unit test
>>cases.  I have it working for the most part, but I ran across a
>>problem trying to use Ruby regexp to find a set of matching curly
>>braces.
>>
>>Please forgive the intrusion of this PHP code onto the list, but I
>>wanted to give you the flavor of what I am attempting to do, that can
>>be easily done with recursive regular expression available in the Perl
>>compatiable regexp engine.
>>
>><php>
>>$test = <<<EOS
>>/* some stuff */
>>class foo {
>>        public \$var;
>>        public function __construct() {}
>>        public function bar() {
>>                if (false) {
>>                }
>>        }
>>
>>}
>>// some other stuff
>>EOS;
>>
>>$re = <<<EOS
>>~(class\s+\w+\s+({((?>[^{}]+)|(?2))*}))~xms
>>EOS;
>>preg_match($re, $test, $match);
>>echo "your class matched:\n", $match[1];
>>
>></php>
>>
>>Now it appears the regexp engine in Ruby does not support recursion
>>(at least in Ruby ruby 1.8.2 (2004-07-29) [i686-linux] that I am
>>working on, and with what I know how to test), thus the only
>>workaround I found was very ugly, model the nesting of braces to a
>>fixed depth, i.e.
>>
>>open = '\{'
>>close = '\}'
>>other = '[^\{\}]*'
>>l1 = other+open+other+close+other
>>l2 = other+open+'('+l1 +')+'+other+close+other
>>l3 = other+open+'('+l2 +')+'+other+close+other
>>l4 = other+open+'('+l3 +')+'+other+close+other
>>l5 = other+open+'('+l4 +')+'+other+close+other
>>re = Regexp.new('class\s+'+@name+'\s+'+open+'((?:'+l5+')|(?:'+l4+')|(?:'+l3+')|(?:'+l2+')|(?:'+l1+')|(?:'+other+
>>'))+'+close, 'ixm')
>>
>>This code did work, but sometimes timed out on valid real classes.
>>
>>I expect I am probably missing some facet of Ruby that eaily allows me
>>to next regexp inside of the Ruby code in some fasion to achieve the
>>result I am looking for, but how to do so eludes me.  Can anyone
>>provide some insight for me on this situation?
> 
> 
> Regular expressions, by all standard definitions, aren't recursive.
> Perl's regexen have been extended to allow it, but it really isn't
> considered a standard regex feature. You might try using a simple
> tokenizer... here's a quick attempt:

I wonder if someone ever looked at the ABNF module on raa. It is a tool 
to generate a parser for a grammar in augmented backus naur form... the 
parser is a Regexp object, so it seem thgere is some general way to 
handle recursive parsing needs..