Re: Negative Lookahead Assertion Problem

Surely trying to match \s* is futile in a negative-lookup? Because \s* can match nothing it will always succeed, thereby never matching?? I tried using \s+.

The following code:

#!/usr/bin/perl -w
use strict;

my @strings = ( "Foo::Bar()",
                "Foo::Bar ()",
                "Foo::Bar   ()",
                "  Foo::Bar()",
                "  Foo::Bar  ()"
                );

my %test = ( 1 => qr/Foo::Bar(?!\s*\()/,
             2 => qr/Foo::Bar(?!\s+\()/
             );
[download]


foreach my $teststring ( @strings )
{
    print( "---$teststring---\n" );
    foreach my $testnum ( sort { $a <=> $b } keys %test )
    {
        if ( $teststring =~ m/$test{$testnum}/ )
        {
            print( "test $testnum matches\n" );
        }
        else
        {
            print( "test $testnum does not match\n" );
        }
    }
}
[download]

produces the following output

---Foo::Bar()---
test 1 does not match
test 2 matches
---Foo::Bar ()---
test 1 does not match
test 2 does not match
---Foo::Bar   ()---
test 1 does not match
test 2 does not match
---  Foo::Bar()---
test 1 does not match
test 2 matches
---  Foo::Bar  ()---
test 1 does not match
test 2 does not match
[download]

Comment on Re: Negative Lookahead Assertion Problem Select or Download Code

Replies are listed 'Best First'.
Re^2: Negative Lookahead Assertion Problem by ysth (Canon) on Jul 19, 2005 at 06:25 UTC
You missed the literal ( after the \s. Together, \s( does specify a meaningful assertion. In your tests, all your sample data is supposed to not match by my understanding of the OP. Only something such as "Foo::Bar is my favorite module" should match.	[reply]
Re^3: Negative Lookahead Assertion Problem by monarch (Priest) on Jul 19, 2005 at 11:39 UTC
I'm confused by your post. Breaking down `qr/Foo::Bar(?!\s+\()/` we get: `Foo::Bar # literal 'Foo::Bar' (?! # start of negative look-ahead \s+ # one or more white space chars \( # literal opening bracket ) # end of negative look-ahead` [download] so to answer your reply, I believe I provided the literal ( after the \s*. To answer your other statement, that something such as "Foo::Bar is my favourite module" should match, I agree - that is how I read the initial requirement from the thread. In which case my answer of using `\s+` kept within the spirit of the original statements made by the thread author. However I wanted to keep my example in line with the example that the author had tried, hence the use of the literal (. Hope this clears up my answer.	[reply] [d/l] [select]
Re^4: Negative Lookahead Assertion Problem by ysth (Canon) on Jul 19, 2005 at 20:12 UTC
My point was that only something like "Foo::Bar is my favorite module" should match, not "Foo::Bar()" and " Foo::Bar()" (which changing * to + make match). As to "missing )", I meant (but put it poorly) in your statement: Surely trying to match \s is futile in a negative-lookup? Because \s* can match nothing it will always succeed, thereby never matching??* The assertion wasn't `\s`, it was `\s\(`, a horse of a completely different color.	[reply] [d/l] [select]
Re^2: Negative Lookahead Assertion Problem by Limbic~Region (Chancellor) on Jul 19, 2005 at 14:07 UTC
monarch, I have to agree with ysth. The \s* is not meaningless because it is followed immediately by a literal (which is what his comment was referring to). To be a complete match it has to match 0 or more whitespaces and an open paren. In your solution there has to be at least 1 space - which doesn't fit the bill. Cheers - L~R	[reply]