use Parse::RecDescent;
use strict;
use warnings;

my $grammar = qq~
   RecTest: ExpSet(s) ( Expression | SubExp )
          | Expression
          | SubExp
          | <error>

   Expression: ELEMENT COMPARE ELEMENT

   ExpSet: ( Expression | SubExp ) JOIN

   SubExp: '(' (ExpSet(s) Expression | Expression) ')'


   ELEMENT: 'h'

   COMPARE: /(?:=(?![><]=))?[><]=?/
          | '!='
          | '='


   JOIN: /and/i
       | /or/i
       | /xor/i


~;

my $parser = new Parse::RecDescent($grammar) or die;
print $parser->RecTest('(h = h and h != h) and ((h > h or h < h') or die;