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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