The /o modifier is used when you only want the pattern compiled once. So in the second pattern, $x will not be incremented and as such, only matches once.

Update: Okay, I promise to read the questions in full before replying from now on (and this time I mean it! ;-). So to answer your actual question...

The difference between qr// and the /o modifier as I understand it is that with /o you can never change the pattern, whereas with qr// you can compile the pattern to be used as part of a larger regex. As the good book shows:

@regexes = ();
for $pattern (@patterns) {
    push @regexes, qr/$pattern/;
}
for $item (@data) {
    for $re (@regexes) {
        if ($item =~ /$re/) { print "Matches!\n"; }
    }
}

[download]

I Hope that's slightly more helpful :).

In reply to Re: Meaning of /o in Regexes by cjf-II
in thread meaning of /o in regexes by mce

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