Multiple newline regex

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

Here's the test case:

while (<DATA>) {
        if (/(\d)\n{2}/) {
                print "$1\n";
        }
}

__DATA__
Line 1
Line 2
Line 3

Line 5
Line 6
Line 7

Line 9
[download]

My question is, why does this not print:

3
7
[download]

As it stands, the script prints nothing. I know there are no \r or \f in the data, and just to be sure I tried matching against \W{2,}.

What am I missing here?

Comment on Multiple newline regex Select or Download Code

Replies are listed 'Best First'.
Re: Multiple newline regex by ikegami (Patriarch) on Aug 31, 2004 at 18:18 UTC
Because you're matching against $_, which only contains a single line of __DATA__ at any given time. Here's a fix: `{ # Read all of __DATA__ into $_: local $/; $_ = <DATA>; } while (/(\d)\n{2}/g) { print "$1\n"; } __DATA__ Line 1 Line 2 Line 3 Line 5 Line 6 Line 7 Line 9` [download]	[reply] [d/l]
Re: Multiple newline regex by Roy Johnson (Monsignor) on Aug 31, 2004 at 18:18 UTC
You're reading one line at a time. Update: here's a fix that reads in paragraph mode. I think this is what you want. `$/=''; while (<DATA>) { print "$1\n" if /(\d)\n\n/; }` [download] Caution: Contents may have been coded under pressure.	[reply] [d/l]
Re: Multiple newline regex by ysth (Canon) on Aug 31, 2004 at 18:19 UTC
You are only reading one line at a time, so you are never going to be able to match against \n{2}. See perlvar on $/ about paragraph mode ($/ = "") or slurp the whole file and loop with //g.	[reply]
Re: Multiple newline regex by Fletch (Bishop) on Aug 31, 2004 at 18:19 UTC
Presuming the default value of `$/` then at any time `<DATA>` is going to have returned after encountering the first newline. The next read will produce a string containing just a single `"\n"`. At no time will `$_` ever hold more than a single newline.	[reply] [d/l] [select]
Re: Multiple newline regex by Velaki (Chaplain) on Aug 31, 2004 at 18:36 UTC
By default, you're reading only one line at a time, so you'll never match something across two lines. Simply prepend your code with `$/=""; # slurp data in paragraph mode, see perlvar for more info` [download] and you'll be able to use your existing code "unaltered". You can do a chock full of interesting things with $/. Take a gander at perlvar and play with the different ways you can read data. Cheers! -v "Perl. There is no substitute."	[reply] [d/l]
Re: Multiple newline regex by rhythmicus (Sexton) on Aug 31, 2004 at 18:27 UTC
Doh! I knew I was missing something very obvious. I've been away from Perl for a while, and I'm just getting back into the swing of things. Thanks for the reminder.	[reply]