From: Jason Sweat <jason.sweat@...>
Date: 2004-11-05T09:04:30+09:00
Subject: recursive brace matching with Ruby regexp

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?

Thanks,

Regards,
Jason
-- 
http://blog.casey-sweat.us/