Balanced delimiter parsing

[moxliukas]

Hello,

I know this question has been raised before, and I did try to Super Search for it, but none of the answers really right on the spot for me.

The problem is as follows:
I am writting a script that parses XML (and no, using XML::Parser or XML::simple is not the options as you need to compile them in order to use them -- otherwise it would be no problem). This XML has some tags that can go inside each other namely in this manner:

<value>...<value>...</value>...</value>

I need to get the stuff between the balanced <value> pairs. Obviously, the (.*?) approach does not work as it selects <code><value>...<value>...</value>. I am suspecting that one might need to use extended regular expression for this (backtracking?), but could you lead me on the rigth track how? (Or is it a much more difficult problem that cannot be solved by only using regexps?)

Thank you in advance

[merlyn]

See XML::SAX::PurePerl for a non-binary Perl XML Parser.

-- Randal L. Schwartz, Perl hacker

[Zaxo]

&Text::Balanced::extract_tagged will do what you want. It is a core module.

After Compline,
Zaxo

[broquaint]

If you want to use XML::Parser and it's ilk then you should just download a binary version of it and copy the the right files to the appropriate places. If however, you can't find a binary version of XML::Parser and you really are stuck up the proverbial creek then you might want to check out the extract_tagged method in Text::Balanced. From the docs:

# Extract the initial substring of $text that is bounded by
# a C<BEGIN>...C<END> pair. Don't allow nested C<BEGIN> tags

($extracted, $remainder) =
    extract_tagged($text,"BEGIN","END",undef,{bad=>["BEGIN"]});


HTH

_________
broquaint

[helgi]

Yours is a Frequently Asked Question. The answer to it can be found in the standard Perl documentation by using the command perldoc -q balanced, also known as:

Can I use Perl regular expressions to match balanced text?

   Although Perl regular expressions are more powerful than
   "mathematical" regular expressions because they feature
   conveniences like backreferences ("\1" and its ilk),  
   they still aren't powerful enough--with the possible 
   exception of bizarre and experimental features in the 
   development-track releases of Perl. You still need to 
   use non-regex techniques to parse balanced text, such as 
   the text enclosed between matching parentheses or 
   braces, for example.

   An elaborate subroutine (for 7-bit ASCII only) to pull 
   out balanced and possibly nested single chars, like "`" 
   and "'", "{"and "}", or "(" and ")" can be found in
            http://www.perl.com/CPAN/authors/id/TOMC/scripts/pull_quot
+es.gz

   The C::Scan module from CPAN contains such subs for   
   internaluse, but they are undocumented.

Further information can be found in perldoc -q How do I remove HTML which despite the name, is directly applicable to XML.

Regards,
Helgi Briem
helgi AT decode DOT is

[Abigail-II]

use Regexp::Common;
qr !$RE{balanced}{-begin => "<value>"}{-end => "</value>"}{-keep}!;

[I0]

Using only regexps


$_="...<value>...<value>...</value>...</value>...";
($re=$_)=~s#((<value>)|(</value>)|.)#${[')','']}[!$3]\Q$1\E${['(','']}
+[!$2]#gs;
print join "\n",eval{/$re/};
die $@ if $@=~/unmatched/i;