Some time ago I was preparing some presentation about Perl and while reading about flip-flop I was wondering how to easily explain how it works. I couldn't. It works in such intuitive way one can tell "get me everything between /start/ and /stop/". But to explain how it does it, you need two pages of paper :) Ok, I exaggerate a little. Anyway, I discovered another useful construction with the same properties: it can be easily described as "take everything between /start/ and (/end/ which is not inside other /start/ .. /end/ block)". Or shorter: "take /start/ .. /end/ block which can include other /start/ .. /end/ blocks", but kids don't try to analyze it at home :) It can be easily extended to two or more levels of nesting. Pity, I didn't discover it before 'beautiful Perl YAPC'... :)
#!perl

while(<DATA>)
{
    if(/start/ .. (!(/start/../end/) && /end/))
        { print }
}
__DATA__
a
a
a
start
b
    start
    c
    d
    e
    end
    f
    start
    g
    h
    i
    end
j
end
k
k
l

[download]

Replies are listed 'Best First'.
Re: Beautiful .. operator
by ikegami (Patriarch) on Oct 28, 2008 at 21:15 UTC

    take everything between /start/ and /end/ which is not inside other /start/ .. /end/ block

    That would be 
    start
b
    f
j
end

    [download]
    but I get 
    start
b
    start
    c
    d
    e
    end
    start
    g
    h
    i
    end

    [download]
[reply]
[d/l]
[select]
      Sorry, my mistake. Original if(/start/ .. (/end/ && !(/start/../end/))) replaced with if(/start/ .. (!(/start/../end/) && /end/)) And description changed to: take everything between /start/ and (/end/ which is not inside other /start/ .. /end/ block)
[reply]
[d/l]
[select]

        take everything between /start/ and (/end/ which is not inside other /start/ .. /end/ block)

        That would be 

        start
b
    f
j
end

        [download]

        but I get 

        start
b
    start
    c
    d
    e
    end
    f
    start
    g
    h
    i
    end
j
end

        [download]

        from /start/ .. (!(/start/../end/) && /end/)

[reply]
[d/l]
[select]
Re: Beautiful .. operator
by Sandy (Curate) on Oct 28, 2008 at 22:34 UTC
    No, it still doesn't do what you advertise: 

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

our $data;

print "\n\n original\n";
foreach (split("\n",$data))
{
    if(/start/ .. (!(/start/../end/) && /end/)) 
        { print "$_\n"}
}

print "\n\n analysis:\n";
print "    - note that range operator is set to true when \$flag1 is t
+rue\n";
print "           becomes false only after \$flag3 has been set to tru
+e\n";
print "\n";
foreach (split("\n",$data))
{
    my $flag1=0;my $flag2 = 0;my $flag3=0;
    $flag1 = 1 if (/start/);
    $flag2 = 1 if (/start/../end/);
    $flag3 = 1 if (!$flag2 && /end/);
    
    print "$flag1, $flag2, $flag3, $_",' 'x(15-length($_));
    
    if(/start/ .. (!(/start/../end/) && /end/)) 
        { print "success\n"}
    else {print "\n";}
}


BEGIN {
    our $data = << 'TEST';
a
a
a
start
b
    start
    c
    d
    e
    end
    f
    start
    g
    h
    i
    end
j
end
k
k
l
TEST
}

    [download]

    result 

    
 original
start
b
    start
    c
    d
    e
    end
    f
    start
    g
    h
    i
    end
j
end


 analysis:
    - note that range operator is set to true when $flag1 is true
           becomes false only after $flag3 has been set to true

0, 0, 0, a
0, 0, 0, a
0, 0, 0, a
1, 1, 0, start          success
0, 1, 0, b              success
1, 1, 0,     start      success
0, 1, 0,     c          success
0, 1, 0,     d          success
0, 1, 0,     e          success
0, 1, 0,     end        success
0, 0, 0,     f          success
1, 1, 0,     start      success
0, 1, 0,     g          success
0, 1, 0,     h          success
0, 1, 0,     i          success
0, 1, 0,     end        success
0, 0, 0, j              success
0, 0, 1, end            success
0, 0, 0, k
0, 0, 0, k
0, 0, 0, l

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