Perl's checking a regex-cache to see if it has already compiled the pattern.

No, the cache is not nearly so general. Each op simply remembers the last pattern it compiled. 

>perl -Mre=debug -e"/$_/ for qw( a a b a )" 2>&1 | find "Compiling"
Compiling REx "a"
Compiling REx "b"
Compiling REx "a"

[download]
>perl -Mre=debug -e"/a/; /a/;" 2>&1 | find "Compiling"
Compiling REx "a"
Compiling REx "a"

[download]

Nothing's stopping you from making your own such cache, though. 

my $compiled_pat = $compiled_pats{$pat} ||= qr/$pat/;

[download]

In reply to Re^2: Optimize a pluggable regex matching mechanism (/o practically deprecated) by ikegami
in thread Optimize a pluggable regex matching mechanism by dredd

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