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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 }

In reply to Re^3: Regex question once-only use of chars in a charset by AnomalousMonk
in thread Regex question once-only use of chars in a charset by gje21c

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