Backreferences

OCTweak has asked for the wisdom of the Perl Monks concerning the following question:

Hi,

On the following code, could someone explain me, why in the first match, the \1 reference, did not block the double quote from being considered part of $2 ?

#!/usr/bin/perl
use strict;
use warnings;

my $line;

print "\n";

$line = '<a href="http://www.mysite.com/" target="_blank">';
$line =~ /\shref=(["'])?([^\s\>\1]+)/; # FIRT MATCH
if (defined $1) {print "\$1= $1\n";}
else {print "1) not defined\n";}
if (defined $2) {print "\$2= $2\n";}
else {print "2) not defined\n";}
    
print "\n"; 

$line =~ /\shref=(["'])?([^\s\>"']+)/; # SECOND MATCH
if (defined $1) {print "\$1= $1\n";}
else {print "1) not defined\n";}
if (defined $2) {print "\$2= $2\n";}
else {print "2) not defined\n";}
[download]

==Program Output
$1= "
$2= http://www.mysite.com/"

$1= "
$2= http://www.mysite.com/

Comment on Backreferences Download Code

Replies are listed 'Best First'.
Re: Backreferences by Abigail-II (Bishop) on Jul 23, 2003 at 15:00 UTC
You can't use `\1` inside a character class. It doesn't have a special meaning. Abigail	[reply]
Re: Backreferences by snax (Hermit) on Jul 23, 2003 at 15:07 UTC
You want: `$line =~ /\shref=(["'])?([^\s\>]+)\1/;` [download] -- you're grabbing the first matched quote character in the parentheses because, as noted above, you can't use the backreference the way you wanted. Once you have the opening quote, though, you can use it as shown in this regex to terminate your match. (edited for clarity)	[reply] [d/l]
Re: Re: Backreferences by Thelonius (Priest) on Jul 23, 2003 at 15:38 UTC
You want: `$line =~ /\shref=(["'])?([^\s\>]+)\1/;` [download] No, that's not quite right, because he wants to find hrefs that are (probably) mistakes, stopping at the first space or > character. Something like this is closer, although I'm sure the regex gurus can improve on it: `$line =~ /\shref=(["'])?(.+?)(?=[\s>]\|\1\|$)/;` [download]	[reply] [d/l] [select]
Re: Backreferences by artist (Parson) on Jul 23, 2003 at 15:06 UTC
\1 as an item in character class doesn't mean anything.	[reply]
Re: Backreferences by Anonymous Monk on Jan 06, 2018 at 07:02 UTC
It seems your else-statements have syntax errors.	[reply]
Re^2: Backreferences by karlgoethebier (Abbot) on Jan 06, 2018 at 19:34 UTC
"...syntax errors." #!/usr/bin/env perl use strict; use warnings; my $line; print "\n"; $line = '<a href="http://www.mysite.com/" target="_blank">'; $line =~ /\shref=(["'])?([^\s\>\1]+)/; # FIRT MATCH if ( defined $1 ) { print "\$1= $1\n"; } else { print "1) not defined\n"; } if ( defined $2 ) { print "\$2= $2\n"; } else { print "2) not defined\n"; } print "\n"; $line =~ /\shref=(["'])?([^\s\>"']+)/; # SECOND MATCH if ( defined $1 ) { print "\$1= $1\n"; } else { print "1) not defined\n"; } if ( defined $2 ) { print "\$2= $2\n"; } else { print "2) not defined\n"; } __END__ karls-mac-mini:playground karl$ perl -c testomato.pl testomato.pl syntax OK [download] Apparently not. How did you jump to this conclusion? ŤThe Crux of the Biscuit is the Apostropheť `perl -MCrypt::CBC -E 'say Crypt::CBC->new(-key=>'kgb',-cipher=>"Blowfish")->decrypt_hex($ENV{KARL});'`Help	[reply] [d/l] [select]
Re^3: Backreferences by soonix (Chancellor) on Jan 07, 2018 at 11:57 UTC
Apparently the AM is referring to the wrong spelling of firth in the comment just before the first `if`	[reply] [d/l]
Re^3: Backreferences by karlgoethebier (Abbot) on Jan 07, 2018 at 12:17 UTC
Ah, yes. See also Firt. Thanks and best regards, Karl ŤThe Crux of the Biscuit is the Apostropheť `perl -MCrypt::CBC -E 'say Crypt::CBC->new(-key=>'kgb',-cipher=>"Blowfish")->decrypt_hex($ENV{KARL});'`Help	[reply] [d/l]