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Lookahead doesn't work

damluar
New User

Jan 9, 2010, 11:55 PM

Post #1 of 7 (6572 views)

Lookahead doesn't work	Can't Post

Hello everyone!
Pls help me out with lookahead. I have strings like "Paris ... 5/5 ..."
I need to find strings that start with "Paris" and don't contain "1/5" and "5/5". I tried this expression:

Code

 $x =~ /\AParis.*(?!1\/5).*/

but it matchs the string above... Can anybody help?

shawnhcorey
Enthusiast

Jan 10, 2010, 5:58 AM

Post #2 of 7 (6570 views)

Re: [damluar] Lookahead doesn't work [In reply to]	Can't Post

Code

#!/usr/bin/perl 
 
use strict; 
use warnings; 
 
while( my $x = <DATA> ){ 
  if( $x !~ /\AParis.*(?=[15]\/5).*/ ){ 
    print $x; 
  } 
} 
 
__DATA__ 
Paris ... 0/5 ... 
Paris ... 1/5 ... 
Paris ... 2/5 ... 
Paris ... 3/5 ... 
Paris ... 4/5 ... 
Paris ... 5/5 ... 
Paris ... 6/5 ... 
Paris ... 7/5 ... 
Paris ... 8/5 ... 
Paris ... 9/5 ...

__END__

I love Perl; it's the only language where you can bless your thingy.

Perl documentation is available at perldoc.perl.org. The list of standard modules and pragmatics is available in perlmodlib.

Get Markup Help. Please note the markup tag of "code".

damluar
New User

Jan 10, 2010, 7:57 AM

Post #3 of 7 (6568 views)

Re: [shawnhcorey] Lookahead doesn't work [In reply to]	Can't Post

Thank you, but I told about negative lookahead, not positive one!

shawnhcorey
Enthusiast

Jan 10, 2010, 8:07 AM

Post #4 of 7 (6565 views)

Re: [damluar] Lookahead doesn't work [In reply to]	Can't Post

Negative lookahead matches if something found does not match. If you have

$x = 'Paris ... 1/5 ...';
$x =~ /\AParis.*(?!1\/5).*/

Then the ' ...' after Paris matches the negative lookahead since a space and three periods does not match 1/5

Negative lookaheads generally do not work if what's before it can match zero times. You need something more definite.

__END__

I love Perl; it's the only language where you can bless your thingy.

Perl documentation is available at perldoc.perl.org. The list of standard modules and pragmatics is available in perlmodlib.

Get Markup Help. Please note the markup tag of "code".

damluar
New User

Jan 10, 2010, 8:26 AM

Post #5 of 7 (6563 views)

Re: [shawnhcorey] Lookahead doesn't work [In reply to]	Can't Post

Thank you for your help. I need to find only strings that contain Paris at the beginning and do not contain 1/5 and 5/5.
I made it this way:
if (/\AParis/){
if ($_ !~ /(5\/5|1\/5)/){
$line_counter++;
print $_, "\n";
};
}

but how can I combine this 2 expressions? I thought that I could use negative lookahead...

(This post was edited by damluar on Jan 10, 2010, 8:26 AM)

shawnhcorey
Enthusiast

Jan 10, 2010, 8:43 AM

Post #6 of 7 (6561 views)

Re: [damluar] Lookahead doesn't work [In reply to]	Can't Post

In Reply To

but how can I combine this 2 expressions? I thought that I could use negative lookahead...

Why? Anyone who has to maintains your code can read what's it's doing as is. Why make things complicated?

__END__

I love Perl; it's the only language where you can bless your thingy.

Perl documentation is available at perldoc.perl.org. The list of standard modules and pragmatics is available in perlmodlib.

Get Markup Help. Please note the markup tag of "code".

damluar
New User

Jan 10, 2010, 9:40 AM

Post #7 of 7 (6557 views)

Re: [shawnhcorey] Lookahead doesn't work [In reply to]	Can't Post

It's a rhetorical question ))) Thank you for your help!

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