in reply to Re^2: Regex problems using '|'
in thread Regex problems using '|'

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).

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Re^4: Regex problems using '|'
by romandas (Pilgrim) on Jul 22, 2008 at 10:50 UTC
    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.
[reply]
      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

      [download]

      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

      [download]

      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.

[reply]
[d/l]
[select]

        I think I see where I'm not being clear in my question. Please bear with me on this, as I don't understand why it isn't working, and I'm trying more for understanding than function. I can always beat at it until it functions; I'd rather understand why it doesn't work the way I expect.. i.e. where are my expectations wrong?

        The record in question is one large string with newlines inside it, right? I'm not sure why the first part of the pattern with alternation would not match the "Remediation Report" and instead use the second half (because it *does* match if there isn't an alternative), unless.. does the regex engine still treat this one large string as multiple strings, separated by the newlines? In other words, why would it skip over a match that works? Does it evaluate each "internal string" in turn?

        I updated the example data above to explain how each record is broken up better. I think the Data::Dumper output was a bit confusing. There is only one vulnerability name in each record, so /g shouldn't apply (I believe).

[reply]

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