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

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In reply to Beautiful .. operator by grizzley

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