Re^2: Regex problems using '|'

[romandas]

Okay, I posted some. What I don't understand is if I just use /Remediation Report\n\n(.+?)\n/, I get the line I'm looking for (Adobe Flash Player Multiple Vulnerabilities - April 2008 - IE), but if I use /Remediation Report\n\n(.+?)\n|^(.+?)\n/, I get the top of the metadata (thread-index: AcjoCau17Ri90HMJR8qoukn2A1g7ng==).

[moritz]

The regex engine starts at the start of the string, and tries to match the first alternative, here Remediation Report\n\n(.+?)\n. It doesn't match, so it tries the second alternative, ^(.+?)\n. That one matches, so it captures the first line in $1.

Without the alternation, the regex engine moves its starting position until it finds the substring Remediation Report.

(Actually it's much smarter than that; it searches for the constant substring with the same techniques that index uses, but from a users point of view that only matters when it comes to speed, not in terms of functionality).

[romandas]

I'm not sure why the match is failing with the alternation when it matches correctly without it. I understand that if the first pattern does not match, it will go on to the second half . That's the behavior I want with regards to the rest of the records. Why is the match failing? That's what I do not understand, since it works correctly as long as the alternation is removed.

[moritz]

If I understood your earlier reply correctly, the regex does match (with the alternation), but it doesn't match the way you want. That's a big difference, and what I tried to explain to you.

That's the behavior I want with regards to the rest of the records.

After looking at the updated data I think that you need two regexes for that:

use strict;
use warnings;

my $str = do { local $/; <DATA> };

if ($str =~ m/Remediation Report\n\n(.+?)\n/g){
    print $1, $/;
    while ($str =~ m/\n\n(.*)\n/g){
        print $1, $/;

    }
}

__DATA__
thread-index: AcjoCau17Ri90HMJR8qoukn2A1g7ng==
MIME-Version: 1.0
# rest of data goes here

The output is:

Adobe Flash Player Multiple Vulnerabilities - April 2008 - IE    
Adobe Flash Player Multiple Vulnerabilities - April 2008 - Mozilla/Ope
+ra
Adobe Reader/Acrobat 8.1.2 and 7.1.0 Update - Acrobat 7.x

The trick is to use the /g-modifier on the first regex although it matches only once. That way pos $str will not be reset, and the next regex match starts where the previous left off.

Also note that ^ will anchor to the start of the string (not to the start of a line) unless the /m modifier is present.