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Re^2: Regex question once-only use of chars in a charset

by elef (Friar)
on May 16, 2011 at 10:52 UTC ( #905041=note: print w/ replies, xml ) Need Help??


in reply to Re: Regex question once-only use of chars in a charset
in thread Regex question once-only use of chars in a charset

What's the o in say if $sorted =~ /^$available$/io;?
I know i is for case-insensitive matching, but I can't find an o modifier in the documentation.


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Re^3: Regex question once-only use of chars in a charset
by AnomalousMonk (Abbot) on May 16, 2011 at 13:05 UTC

    The  /o modifier means "compile once" for a regex it modifies. Consider these examples:

    >perl -wMstrict -le "my $s = '1a2b3c'; ;; print qq{no /o}; for my $i (qw(3 2 1)) { print qq{matched '$1'} if $s =~ m{ ($i.) }xms; } ;; print qq{with /o}; for my $i (qw(3 2 1)) { print qq{matched '$1'} if $s =~ m{ ($i.) }xmso; } " no /o matched '3c' matched '2b' matched '1a' with /o matched '3c' matched '3c' matched '3c'

    The function of the  /o modifier has been generally replaced by the qr// regex object builder (see in perlop).

    I was a bit surprised not to see anything about  /o in perlre, but it is (briefly and obliquely) discussed in qr/STRING/msixpodual (5.14), and the following remains in perlretut (at least through 5.12):

    Optimizing pattern evaluation
    We pointed out earlier that variables in regexps are substituted before the regexp is evaluated:
    $pattern = 'Seuss'; while (<>) { print if /$pattern/; }
    This will print any lines containing the word "Seuss". It is not as efficient as it could be, however, because Perl has to re-evaluate (or compile) $pattern each time through the loop. If $pattern won't be changing over the lifetime of the script, we can add the "//o" modifier, which directs Perl to only perform variable substitutions once:
    #!/usr/bin/perl # Improved simple_grep $regexp = shift; while (<>) { print if /$regexp/o; # a good deal faster }

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