Re: RegEx -- better one ?

[Rob Thorpe]

[EMAIL PROTECTED] wrote:
Hi all,

i make a interpreter for a query string. As a result i need something like:
'true'. or sometimes 'false' :-)) 

my input looks like:
((exp) and (exp)) and ((exp) or (exp))
i started with this script:
----
#!/usr/bin/perl
$reg = "(\\([^()]*\\))";
$str = "((a=1 )or(a =2))and(((a= 3)and(a = 4))or(  (a= 5)and(a= 6)))";
print "$reg\n";
while($str =~ /$reg/g){
        $match = $1;
        $match =~ s/\(/\\\(/;
        $match =~ s/\)/\\\)/;
        print "$match\n";
        $str =~ s/$match/ true /;
        print "$str\n";
        sleep(1);
}
exit;
------
is there a better regexp for this ? and what is the best way take tare to 
the 'and' 'or' things ???

Note: I do this for a scratch. at the end i need a java code, but i am 
better in perl for design, so i done it in perl first. I read something, that 
the perl regex is more powerfull than others.... so i must take care about 
this. Filaly it has to work in Java.
This is a job for a parser.  You could use Parse::RecDec as Eugene 
Haimov pointed out, but I'm not sure there is a comparable library for 
Java.  It certainly can't be done using regular expressions.
You are parsing a grammar something like:-

expr := factor
       | factor 'AND' expr
       | factor 'OR' expr
factor := ( expr )
       | something
and the something is the "a= 2" bit that you haven't really defined.

my $next;

sub factor()
{
  if ($next = '(') {
    lex();
    expr();
    lex();
  } else {
    something();
  }
}
sub expr()
{
  factor();
  if($next = 'and' || $next = 'or') {
    # code to deal with and & or goes here.
    lex();
    expr();
  }
}
sub lex()
{
  my $ret = $next;
  if ($_ =~ s/^\(//)   { $next =  "("; }
  elsif ($_ =~ s/^\)//)   { $next =  "("; }
  elsif ($_ =~ s/^or|^OR//)   { $next =  "or"; }
  elsif ($_ =~ s/^and|^AND//)  { $next =  "and"; }
  elsif ($_ =~ s/^\w+//)   {$next =  "x"; }
  return $ret;
}
This is of course completely untested.



THANX FOR HELP ANS COMMENTS ON MY SNIPE. 
Kris
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--
Robert Thorpe
Software Engineer
Antenova Limited
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