regex: negative lookahead

Hi,

I'm confused with a negative lookahead. Here's some example code. I want it to match on $needs and to not match on $has. The examples 3 and 4 are only as a reference to a fixed string, but I basically want ^($start(.*?))(?!$end)$. Any ideas?

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

my $start = "log4j.rootLogger=";
my $startlong = "log4j.rootLogger=INFO, FILE";
my $end = ", SYSLOG";

my $needs = 'log4j.rootLogger=INFO, FILE';
my $has = $needs . ', SYSLOG';

print "needs_matched $needs\n" if $needs =~ /^($start(\w+(,\s)?)+)(?!$
+end)$/;
print "has_matched $has\n" if $has =~ /^($start(\w+(,\s)?)+)(?!$end)$/
+;
print "needs_matched $needs\n" if $needs =~ /^(log4j.rootLogger=INFO, 
+FILE)(?!, SYSLOG)$/;
print "has_matched $has\n" if $has =~ /^(log4j.rootLogger=INFO, FILE)(
+?!, SYSLOG)$/;
[download]

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Re: regex: negative lookahead by kennethk (Monsignor) on Dec 05, 2011 at 15:54 UTC
If you take a look at what you have in $1 after your failed miss for 2, the issue will become more apparent, I think; `print $1;` yields `log4j.rootLogger=INFO, FILE, SYSLOG`. Essentially, you have already consumed "`, SYSLOG`" by the time you get to the end of the string, so clearly that bit does not follow. A negative look-behind will actually yield what you intended: `print "has_matched $has\n" if $has =~ /^($start(\w+(,\s)?)+)(?<!$end)$/;`	[reply] [d/l] [select]
Re^2: regex: negative lookahead by svenXY (Deacon) on Dec 05, 2011 at 16:08 UTC
Hi, ++kennethk - thanks for clarifying. I'm still a little bit confused by the difference between (?!...) and (?<!...) though. Regards, svenXY	[reply]
Re^3: regex: negative lookahead by kennethk (Monsignor) on Dec 05, 2011 at 16:38 UTC
The difference is a lookahead versus a lookbehind. If you ignore the lookahead/lookbehind because they are zero width (don't consume characters), you would expect `/^($start(\w+(,\s)?)+)$/` to grab an entire string between start and finish. When you get to the part of the regular expression where you have your assertion, the cursor is here: `log4j.rootLogger=INFO, FILE, SYSLOG ^` [download] If you look ahead of this position, there is nothing, which clearly passes the negative lookahead check. The offending string is behind your position, thus you need a negative lookbehind.	[reply] [d/l] [select]
Re: regex: negative lookahead by Khen1950fx (Canon) on Dec 05, 2011 at 20:50 UTC
I've never had a the need to use a lookaround, but your question piqued my curiosity. As an experiment, I used Regexp::Assemble to rework your example because it just does the right thing with assertions, lookaheads, and lookbehinds. `#! /usr/bin/perl -slw use strict; use Regexp::Assemble; my $needs = Regexp::Assemble->new->add( qw[ FILE INFO ] ); my $has = Regexp::Assemble->new->add( qw[ FILE INFO SYSLOG ] ); while( defined( $_ = <DATA> )) { chomp; if( /($needs)/ ) { print $needs->as_string; } } while( defined( $_ = <DATA> )) { chomp; if( /($has)/ ) { print $has->as_string } } __DATA__ FILE INFO SYSLOG` [download] I believe, if I"ve understood you correctly, that it works the way that it should. What do you think?	[reply] [d/l]
Re: regex: negative lookahead by vinian (Beadle) on Dec 06, 2011 at 05:01 UTC
i don't think there is something to do with regex negative lookahead, the problem lies in the `(\w+(,\s)?)+` it will match "INFO, FILE, SYSLOG", but not "INFO, FILE". "+" is greedy, it eaten whatever match by "( ... )" . I don't how to make the above regex match less, but i use this `print "has_matched $has\n" if $has =~ /^($start(\w+(,\s)?)+?)$(?<!$end)/;` and it will not match `$has`.	[reply] [d/l] [select]


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regex: negative lookahead

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