Re: Odd behaviour with Regular Expressions

Paul,

Consider the following example:

===============================
use Parse::RecDescent;

my $grammar = <<'TEXT';
ident :  /^(?!(return|exit|if))[A-Za-z0-9_]*/
{
     print "pattern matched OK\n";
     print $item[1];
}
TEXT

my $p = new Parse::RecDescent($grammar);
$p->ident('test if');
================================

This outputs

================================
pattern matched OK
test
================================

as you expect, so you're on the right track.  Perhaps the '*' is causing ident 
to match the empty string ('').  For example, this

   $p->ident('.test if');

outputs

=================================
pattern matched OK

=================================

-davidm

PerlDiscuss - Perl Newsgroups and mailing lists wrote:
> I am attempting to create a rule that will match any string apart from
> those that are keywords in my language.
> 
> I have developed the following regular expression, which acheives this,
> but, when trying to access a word that is accepted by the regular
> expression, via $item[1], nothing is returned ( I have also tried
> $item[0], $item[2] etc). I have placed a print statement in as a debugging
> aid, to prove that the rule works properly debugging aid:
> 
> When entering a keyword, such as "return", as expected, nothing is
> printed. However, when entering a non-keyword, such as "foo" 
> only the string "pattern matched OK" is printed, and the value of the
> matched word ($item[1]) is not printed.
> 
> Can anyone tell me where I am going wrong here? 
> 
> Followin is the  offending piece of code
> 
> Paul Kennerley
> 
> ident :  /^(?!(return|exit|if))[A-Za-z0-9_]*/
> {
>     print "pattern matched OK\n";
>     print $item[1];
> }
> 
>